One Sunday last month, just as the sun cleared the Vosges mountains, I stepped into the Lac de Longemer in north-eastern France. The water smelled faintly of falling leaves, sprouting ceps, wet feathers. Chub and perch cruised the shallows, hunting their own fry.

I kicked out and headed south, spruce forests on my left, campsites on my right, towards the pink-washed hotel at the far end of the lake. Breaststroke, backstroke, crawl: a hundred of this, a hundred of that, up and down, up and down . . . by the time I left Longemer, the shores were full of sunbathers and picnickers. I'd done four lengths and covered more than 7km.

Eight or nine months ago, anyone who knew me would have laughed themselves sick at the thought of me swimming for three hours without a break. I was 45 and I hadn't even been in a pool for years. Not only was I clinically obese, at 178cm tall and 110kg (5ft 10in and 17st 4lb); I was in denial. I had broken a neighbour's bench just by sitting on it. My mother would nod at my stomach and ask when "it" was due. One so-called friend described me as a "no-good greedy chubster", while another shouted "Thar he blows!" when I joined him on a beach. Meanwhile, I told myself I just needed to lose a few pounds, and it was normal for my thighs to rub holes in my jeans.

I'm still officially overweight, but you'd have to find something else to tease me about. At 88kg (13st 12lb), I'm 22kg (3st 7lb) lighter than I was, and my weight is still inching down. I've had to drill more holes in my belt, and if ever I forget it my trousers are likely to end up around my ankles. I am fitter than I have been for decades. I have buns of iron and only the smallest of man-boobs, and I no longer feel waves of melancholy every time I see a photo of my slimmer twentysomething self. I am lucky enough to live in France, in the Vosges, with their silent forests and trout-filled lakes, and most days, if the thunderstorms hold off, I walk for at least four hours, or swim or mountain bike for two. Meanwhile, everyone I know back in Britain seems to be getting unhealthier by the day.

Have I been on a diet? People have been asking me that for months. Sometimes, I'm ashamed to say, I joke that no, I'm terminally ill. I don't know what the straight answer is. You think of a diet as something that hurts – and I'm enjoying the business of feeding myself more than I ever did. Not once this year have I left the table feeling hungry or hard done-by. I have not signed up for WeightWatchers, come over all Atkins, discovered the delights of acai berries, or switched to the new "non-fat" fat. I have not – though God knows I should have – significantly reduced my boozing. I have stopped eating almost everything that I once thought of as food, but I'm loving what's replaced it.

Towards the end of last year, I had a couple of worrying cholesterol tests. Good cholesterol and bad cholesterol, HDLs, LDLs, triglycerides . . . the details are tedious, but the bottom line was this: my cholesterol was too high, and it had to come down. There is a strong link between high cholesterol and cardiovascular disease, heart attacks and strokes. I needed to get my body in order by eating more healthily and losing some weight; if not, my doctor would be prescribing a lifetime's supply of the cholesterol-lowering drugs known as statins.

I didn't fancy that. Before I knew it, I'd be carting around a pill dispenser and fussing about whether I'd taken my meds. One day, no doubt – but not right now, thanks.

But what does eating healthily mean for someone with cholesterol trouble? Mostly, I discovered, it's the same as it is for everyone: eat a rainbow, get your five a day, bump up the roughage and cut down on the salt. Leave that bordeaux on the supermarket shelf, you disgusting cirrhotic pig. Replace every fat you can think of with extra-virgin olive oil and rapeseed. Fruit and especially veg good, biscuits and burgers bad. But it particularly means avoiding saturated fat. After eight months of dodging it, I still can't tell you exactly what this is, but thanks to a book called Cholesterol: Food Facts and Recipes, I do know where you'll find it. "Foods high in saturates," Juliette Kellow tells us, "include fatty meats, full-fat dairy products, butter, lard, cream, cheese, ghee and coconut and palm oil. Many processed and takeaway foods can also be high in saturates." That last bit's a bit of a wimp-out, mind. Five minutes checking "traffic light" labelling in any supermarket will show what a shocking proportion of ready meals rate red or amber for saturated fat. The recommended daily limit is just 30g for a man and 20g for a woman, and the ease with which the lazy or time-deprived cook can exceed these levels is frightening. You need to keep an eye out for trans fats, too; they're often labelled as hydrogenated fats or hydrogenated vegetable oil. You might almost think the manufacturers were trying to hide something.

So: no more cakes, pastries, pasties, biccies, shop-bought pizzas, sausage sarnies and hot dogs. Arrivederci spag bol, cordon bleu chicken, cheese toasties. Adieu brie, cheddar, camembert, roquefort and Devonshire cream. Goodbye, in fact, to 95% of my weekly shop.

What was left? As far as I could tell: fish (the oilier the better), the odd bit of lean meat, an egg every now and again, skimmed milk, tofu, soy milk and other such muck, and as much fruit, veg, pasta, porridge, bread and rice as I could cram in, the more wholemeal the better.

Not only did I now have to cook almost everything from scratch, but all my staple ingredients had been taken away. Could you really make pasta sauce without bacon? Serve it without parmesan? There was no point asking the neighbours. My bit of the Vosges lies within Lorraine, whose cuisine, the Larousse Gastronomique cookery encyclopedia notes, "is based principally on pork". Regional specialies include "the famous quiche . . . marinated meats baked in a piecrust with egg custard . . . the andouillettes of Epinal, the black puddings of Nancy, the sauerkraut of Saint-Dié, fresh pork liver, suckling pig in aspic . . ." The 400-word entry mentions sweetcorn and cabbage, but that's your lot for vegetables.

What might have been an ordeal, however, turned out to be a doddle. Since meat and dairy were largely off the menu, I asked a vegan friend how vegans cook. You start by frying an onion, she told me as she sipped her fennel tea, then you add more vegetables and some beans. I reckoned I could manage that. What about soup, I asked one of the Guardian's food experts. You start by frying an onion, he shouted above the screams of boiling lobsters, then you add more vegetables and some liquid. I already knew how to make a salad.

There was a little more to it than that, of course, and three books in particular have made the mountain easier to climb: Healthy Eating for Lower Cholesterol, by Daniel Green and Catherine Collins; Classic Light from Jeni Wright and Le Cordon Bleu Chefs; and The Low Fat Low Cholesterol Cookbook by Christine France. The recipes are accessible rather than aspirational, but it would take a long time to get bored of dishes such as rabbit with mustard and leeks, or white truffle and mushroom risotto. Green and Collins's book, meanwhile, has taught me more about nutrition than any official handout.

The big problem, it turned out, was not what to cook, but remembering to cook it. If you want homemade tomato and courgette soup at 1pm, I learned, you can't start thinking about it at 12.50. Fancy a pizza? You should have begun the dough a couple of hours ago, or made one earlier and frozen it. Even now, lunchtime often finds me staring into an empty fridge, cursing my own stupidity. Dinner? Sometimes I don't manage it until 10pm. And I can hardly blame my busy schedule: I live on my own, and as a journalist I can largely choose my own hours. Like a lot of people, I also find myself with less work to distract me. Thanks, global financial meltdown.

Still, the switch has been far easier than I imagined. I have the occasional longing for cheese, from industrial British cheddar to the stinky munster they make just down the road, and the smell of sausage torments me whenever I pass a barbecue. But the pleasure of getting to grips with once-scorned ingredients more than compensates. On a hot summer day it's hard to imagine anything better than avocado, tomato and fennel salad, sharpened up perhaps by some home-grown carrot. At this time of year, most of the ingredients will come from the vegetable garden I started in April, as soon as the snow had melted.

Living on my own has made it easier to improvise around recipes rather than follow them to the letter. If the worst comes to the worst, I'm the only one who suffers. I've had some surprises, but only one meal was too boring to finish. Meanwhile, a few of the dishes I've blundered into, like bread with mixed seeds and home-dried onions, are good enough that friends and family ask for them when they visit.

No one, by the way, has complained that the onion bread is lacking in salt, though I halved the amount most recipes would call for. That's been another revelation: you can slash the salt in most meals – even cut it out entirely – and as long as you go heavy on the herbs or other flavourings only the most hardened addicts will notice. And as they're the sort who season their food before they've even tasted it, they'll simply assume they've been too light-handed with the shaker.

Do I worry about reverting to my old ways? Not really. Right at the beginning, I decided the rules would not apply when I was feeding other people, or sitting down at their table. I would not deprive my guests of a ripe brie, or turn my nose up at a lamb chop because of its lovely smoky, melty fat. This article notwithstanding, I didn't want to become known as a diet bore. But that social consideration has turned out to be a safety valve. If ever I find myself dreaming of spicy merguez, I tell myself that sooner or later someone will cook me one.

I know some inspired cooks, and I've taken full advantage of their hospitality this year. I've enjoyed every mouthful they've served up – but afterwards I've had no difficulty in getting back into my own routine. Nor, when house guests have left, have I had any trouble chucking out the uneaten lard and oven chips.

Given that many people on a low-fat diet are supposed to be losing weight, is there any need to count calories? Unless you have a ridiculously sweet tooth – by which I mean you'll eat sugar or honey by the spoonful – I'd say not. Biscuits are out, remember, because of their fat content. So too are gateaux, milk chocolate, ice cream, clotted cream, individually wrapped Dutch cheeses, sausage rolls, BLTs, chicken tikka masala, half-pounders with bacon and cheese. How can you not lose weight if you're not eating those?

If you're drinking too much, that will naturally make it harder to shift the pounds. But how do you cut down the booze? Ask me in six months or so. With French wine so good and Belgian beer so cheap, this seems to be an area where willpower is called for. And that's one muscle I haven't exercised recently. I knock back a bit less than I did, but still far more than three or four units a day. This is, I'm reluctantly coming to realise, the Next Big Health Challenge.

How much of my own weight loss is down to exercise I couldn't say. The pounds started falling away almost as soon as I stopped eating crap, and my early exercise regime was much less hardcore than it has become. It feels more like I've been caught up in a virtuous circle, with the weight loss making it more enjoyable to take exercise, the increased exercise making it easier to lose weight, and so on.

And the cholesterol? I had another blood test in April, and got a big double thumbs-up. My levels weren't just better; they were good. The doctor sounded surprised, and so was I. Not that things had improved – it would have been a miracle if they hadn't – but by how much. The British Heart Foundation says that healthy eating "can help reduce your cholesterol levels by over 10%" (which doesn't sound like a lot for anyone with serious problems) but mine had fallen a great deal more. One GP has since told me he asks his patients to change their diet but usually ends up prescribing statins anyway. Another admitted he'd only known one person for whom diet alone was enough.

Why was I an exception? The only explanation I can find is that I was able to take the recommendations more seriously than most – not because I'm unusually virtuous but because of my lifestyle and the pleasure I found in the new regime. I live on my own, doing a job where I can choose my own hours and set my own routine. My daughter has grown up and has her own flat; I have no partner to feed or be fed by. I have the space to grow my own vegetables and even a pond to raise a few fish. Step out of my front door and I am in countryside that just cries out to be walked, cycled or swum through. But I'm not going to beat up on myself either. There are plenty of other idle single people who moan about their health but never quite get round to doing anything about it. I did, and I think I have the right to be proud of myself.

I've done a lot of thinking over the past few months, much of it when I was chopping up vegetables. But it's in Longemer that my brain has really had a workout. I've heard swimmers talk of zoning out when they're doing distance work, and, yes, you can do that in a pool, with a lane to yourself, no hazards, just the stroke, stroke, flip, stroke, stroke, stroke up and down. You can't – well, I can't – zone out in a stretch of water almost as long as Oxford Street, where the wind can pour a wave down your throat as you're trying to take a breath, any kind of debris could be floating just below the surface and the most stupid images pop into your head no matter how you try to keep them out. I haven't once swum in Longemer without imagining drowned bodies in the shallows and hungry fish rising from the depths. The lake is more than 30m deep – enough to drown a 10-storey building – and legend has it that Charlemagne himself once pulled out a pike as big as a six-year-old.

The first time I swam just a width, with barely 300m to cross from meadow to mountain, I wondered if I might just expire halfway, sink down to the ooze and waterlogged trees and Christ knows what else. There must be some reason no one else was more than a few strokes from shore. Now I worry that one day I might simply let myself sink. Those dark depths are both terrifying and enticing.

When I'm not being such a wuss, I think about the man I'm turning into. Since my late 30s I have fretted pointlessly, life-sappingly, about growing old – not about death but about the possibility of decades of infirmity. Meanwhile, of course, I was doing everything to make that more likely – furring up my arteries, piling on the pounds, filling my body with junk. One day in the lake, I realised that while you can't stop yourself ageing, or shield yourself from accidents or infection, you can at least try to shape your old age. Every now and again, when I'm walking down some forest track, I will come across some 80-year-old codger splitting firewood or digging over his vegetable patch. It took me a while to realise it, but I have a new ambition. I am going to become one of those sunburned, wiry old bastards who look like they just might live for ever •