'Fell running," says Matthew Robinson, striding up a wall-steep hill with the ease of a funicular train, "is an extreme sport." Until recently, he says, it was considered one for the Kendal Mint Cake crowd, pin-thin middle-agers who got their kicks scaling the country's highest peaks. Not any more. Forget kite-surfing, base-jumping and all your other newfangled, boundary-pushing pursuits: "These days, you're seeing younger people coming through, tempted by the buzz that fell running provides."

Never mind the next high, I think. Let's just get to the top of this hump and hope there isn't another one playing a cruel trick by hiding behind it. We have hardly got 30m above Wordsworth's cottage and already I'm wheezing like an old-fashioned kettle.

It is Christmas Eve and we are in Ambleside in the Lake District. Crack personal trainer Robinson, who for the next 12 weeks will be bringing his get-fit plan to G2, is showing me how to shape up without the gym and breathe new life into a tired exercise regime. He is damned if we're not going to run right to the top of the 2513ft (766m) Great Rigg.

The lakes, it would seem, is where professionally fit people come to test themselves. Robinson himself is toying with attempting the notorious Bob Graham Round, in which terrifyingly trim lunatics try to top 42 Lakeland peaks in less than 24 hours, running 61 miles and climbing the equivalent of Everest while they are at it. It occurs to me that I have never run up a hill before, unless you count the gentle incline of the Millennium Bridge by the Tate Modern. I am clearly out of my depth.

I point to my girly pink-and-white Nikes and try the old "but I've not got the right shoes" excuse, but Robinson is having none of it. "Even in your city-slicker trainers you'll be all right, as long as we take it easy coming down," he counsels, in exactly the kind-but-ever-so-slightly strict tone every personal trainer needs.

"When I did my first fell race I thought I was going to die halfway through it," he says cheerfully, charging ahead through some dense bracken. "You'll feel this tomorrow." From the pain in my thighs that much is already sorely clear: when I wake up on Christmas Day I will feel like the Tin Man before Dorothy showed up.

After a minor miracle that sees me half run (OK, one-sixth run) up the vertiginous slopes, I somehow drag myself to the top. As we survey the panorama over Lake Windermere and I decide this fell-running lark might be for me after all, Robinson gees me up with tales of his past - when it was impossible for him to run anywhere.

"I used to be fat, really fat," he says. "So fat that when I ran, my thighs rubbed together, burning, as if they were trying to start a fire. I went to the doctor with headaches, thinking it was because I needed glasses and he measured my blood pressure. It had gone through the roof. He said that if I carried on as I was, I was heading for heart trouble and an early grave."

Robinson was 13 and weighed 14 stone, exactly the same as his 40-year-old father. Later, he shows me a picture of himself, taken around that time. Robinson looks like a giant baby, rolls of fat gathered above his swimming trunks like fleshy Hula Hoops.

The doctor's harsh words gave Robinson a jolt. The next week, he went to rugby practice. Then he started lifting weights. And playing basketball. And doing sit-ups. Then came running, and then the realisation that he wanted fitness to be a big part of his life for ever. A degree in sport and exercise science later, he qualified as a personal trainer and has been operating in the Lancaster area ever since.

"I don't want to float my own boat too much, but I think I inspire people to get fit because I was such an unhealthy disaster," he says. "The odds were stacked against me. I don't have great genetics - neither of my parents are at all sporty and a lot of my mum's family from Trinidad are obese. If I stopped working out I would get overweight quite quickly, whereas I know people who naturally look fantastic despite eating crap and doing no exercise. It's about getting good, regular exercise, not starving yourself or eating strange concoctions."

It might not have the allure of those myriad miracle plans you'll see at supermarket carousels at this time of year (The Once-A-Week workout and other too-good-to-be-true titled tomes) but Robinson's no-nonsense regime works, and its architect is hulking, healthy and very buff proof of it.

The plan he has masterminded for G2 is gloriously simple - get your heart rate up by doing some cardiovascular exercise at least every other day, incorporate daily toning exercises (there will be a new one each week, so by the end you will have a repertoire of 12 to mix and match), don't eat like an idiot, drink plenty of water (tea and coffee doesn't count, and he is very strict about that) and you will never have felt better.

There is no dieting, no fiddly instructions to follow, no unsubstantiated promises, just a set of foolproof suggestions for sorting out your fitness. It's suitable for men and women of all ages and abilities, and if you are finding it all a bit easy, he will give suggestions for upping the ante. At the end of the column there is a section with hints for getting through the coming week and some pointers on how your body should be changing.

A lot of exercise programmes these days, especially those on video or DVD, are fronted by a celebrity toned to within a micromillimetre of their showbiz lives. The message behind each seems tantalisingly simple: follow their advice and you too will have a calendar in HMV by next Christmas. The principle, says Robinson, is flawed.

"Clients come to me saying I want to look like so-and-so or I want arms like Madonna so I say, OK, first you need the right genetic make-up, then prepare yourself to train at least a couple of hours a day every day for the rest of your life, and you're sorted."

Robinson has no time for unrealistic guff, however, his aim is solely to help people realise their goals, whether that is running their first 10k, training for the marathon or simply getting a nicer bottom. "One of my clients was in his 70s and just wanted to know whether being a bit more mobile would improve his life, and it did," he says. "Exercise can do whatever you want it to, to a certain extent. I firmly believe anyone can improve their current state."

After a day out on the fells with him, several things become clear: one, Robinson is double hard. Two, I'm not. Third, he is a bloody good personal trainer. Not just because he knows his stuff and manages to tell me I have a saggy bottom in such a way that I don't want to maim him afterwards (my weak gluteus minimus is allowing my thighs to turn inwards rather than straight, leading to some dangerous over-pronation, apparently), but because he's fun, funny and knows what it's like to be a bit of a dumpling too. And as we race down Great Rigg, me taking ginger steps for fear of falling off and him launching himself down like a barrel over Niagara Falls, the last thing becomes apparent: he's clearly a maniac. Which you would have to be, right, to choose to work out for a living?