It is a damp, grey morning in south London and I am outdoors, still half asleep. The wind cuts like a knife and all I am wearing is shorts, tights and a polyester jersey. My muscles have clenched from the cold and no amount of stretching or rubbing will loosen them. But I tighten my laces and start to run. Uphill, as if things weren’t bad enough. My body jars with every step. Before I have gone 100 metres, I am thinking about packing it in. But no: if I give up today, maybe I won’t even get this far next time. So on I plod. Thud thud thud, trudge trudge trudge, up through Norwood Park, past the swings and skateboard ramps, through the boggy, doggy hollow, then left along the main road towards the Crystal Palace transmitter. I gain a little more height before peeling off down a side street to windswept park number two.

Beyond are more ups and downs, followed by the long, gentle decline of Beulah Hill

Four kilometres in, I am dripping with sweat. I push up my sleeves and try to forget that, because of the way I have looped back on myself, home is just a short walk away. I focus on the positives (I am almost halfway through; the bits that were hurting at the start aren’t hurting any more) and do my best to ignore the negatives (I am not halfway through; the bits that weren’t hurting at the start are hurting now).

On it goes – more quiet streets and silent despair. Somehow, I make it home, to a cup of tea and a sausage sandwich.

“How was it?’” my wife asks, breathing from the side of her mouth so that she doesn’t have to smell me.

“It was all right,” I say, thinking: “It was hell,” but knowing I will be doing it again. And, secretly, both dreading it and looking forward to it.

I have been running regularly since early 2014, when I decided I had to do something about my ever-expanding gut. After moving to London from the French mountains, where wild swimming and hiking had kept me fit, I had found the pounds sneaking back on. Although I had never once run for pleasure, I liked the idea of an activity that was cheaper than gym membership, could be done almost anywhere and fit easily into the weekly routine.

I probably wouldn’t have managed it without the NHS’s couch-to-5k plan, a set of free podcasts that use cheesy pep-talks and detailed, real-time instructions to guide you through a series of gradually lengthening runs. Although I had barely run since I was a schoolboy, the podcasts’ gently-gently approach made the transition as painless as possible. I occasionally got breathless, but I never felt as if I was being pushed too hard. By the ninth and final week, I was just about capable of running 5km without a break, which seemed pretty good for an overweight fiftysomething. A year later, I was into double figures and running every two or three days.

Little by little, the distance has crept up. I now run about five times a week, totalling 40-45km. I have done it in London and Barcelona, Cornwall and Moselle, Dunbar and County Durham, down city streets and dirt tracks, on mountaintops and marshes. If I can’t get out first thing in the morning, I will go for a “runch” at work. My shortest regular route is 5km, through the wooded hills of Dulwich and Sydenham, the longest a flat 14km to the Guardian offices in King’s Cross. I have raced in two half-marathons and one full.

I am not the fastest thing on two legs: it takes me five or six minutes to cover a kilometre, nine or 10 for a mile. The New Forest marathon took an embarrassing five and a bit hours, not least because I ran out of steam and ended up walking some of it. The only reason I can imagine for doing another is to prove to myself that I can run the whole 42.2km. I am, however, a lot fitter and slimmer than I used to be – down from 100-odd kilograms to 84. It is not all because of running – I have cut down on the cakes, chocolate, biscuits and booze and even done a bit of Weight Watchers – but running has definitely helped. It has built muscle and stamina, too. I will never be “ripped”, but I am in better shape (in all senses) than I have been since my 20s.

This may sound like bragging, but I need to remind myself why I do what I do. Sometimes I enjoy running, but mostly I endure it. I frequently hate it. As for the much-touted “runner’s high”, the closest I come most days is a panted: “Thank Christ that’s over.”

Because, above all, running is hard work. To put the full horror into words, you have to stick one foot in front of the other, again and again and again. On a good day, running just happens; on a bad day, every step must be willed into existence. On my most recent outing – a joyless slog through Islington and Hackney – I had to bully my legs almost 6,000 times. That is four pleas of “Again, you bastard” for every word in this article.

It only takes two or three bad runs in a row to feel as if you are not getting anything out of it. Running can be boring, too, especially when you are pressed for time or short of ideas and just do a circuit you have done 100 times before – past the same houses, down the same streets, thinking the same thoughts. Every now and again, I just give up, mostly with the words: “Sod it, I can’t be arsed,” rather than: “Sod it, this is too painful.” I tell myself that the feeling will pass, and usually it does, but sometimes it persists until the very last step.

For me, at least, running successfully is about psychology as much as physique. So, I have learned to do everything I can to shake things up, from listening to podcasts, audiobooks and music (nothing gets me moving like Agatha Christie or the KLF) to trying new routes constantly. There is a wonderful app called RunGo that lets you map out a path, then provides turn-by-turn directions through your headphones. Without it, I would either be running up and down the same main roads or getting lost down side streets, stopping to work out where I was, then trying to remotivate myself to run. With it, I can happily navigate my way across London or through a foreign forest.

This doesn’t do anything for the hypochondria, unfortunately. I often feel a little discomfort – tired muscles, too-tight tendons – early in a run, although this usually passes as I get into the rhythm. I suspect its psychosomatic, with my body offering an excuse to cut things short.

But that doesn’t mean you can afford to ignore it. Sometimes you really do damage yourself. I have been lucky so far: my only real injury was a couple of years ago, when my hips started to hurt after a string of long runs. It turned out that I had been pushing my muscles too hard before they had had time to adapt; a few weeks of physio put things right. Other than that, I am happy to say that I have never felt better. I am pretty sure that the next time I injure myself it will be by tripping on a tree root, slipping on ice or trusting a car to stop at a zebra crossing.

Not that this stops non-runners telling me I am doing irreparable damage to my hips, knees, ankles and heart. Although study after study has shown that running is good for your joints and can extend your life by several years, many armchair experts will not be told. It is hard not to think that at least some are looking to justify their own indolence.

So, why do I keep running, when my mind and body and so many other people tell me not to? Two reasons. First, when I am not actually pounding the pavement, I am pretty clear about the good it is doing me. When everyone around you is getting just a little bit chubbier and a little more out of breath, there is undeniably a thrill to seeing your own stomach getting flatter and your endurance increasing. Also, I am in a better mood when I have run. After two days of idleness, I get restless and irritable. I am not sure if this is my default state and running relieves it or if I am now so addicted that I get withdrawal symptoms when I stop, but the result is the same.

I am happy with my own company and I normally run without a partner. I relish the chance to be alone with my thoughts, even if they are mostly about how uncomfortable I am.

Plus, every now and again, maybe once or twice a month, I love, love, love the experience – enough to make up for all the horrors that have gone before. I am not too hot and not too cold, just the right amount of tired, with the feeling I could run for ever – and then nature gives another nudge. It might be early on a December morning, with dawn gilding the eastern horizon, or midday in August, with rabbits scattering across a field. Once it was in Catford, with a thunderstorm soaking my clothes and washing the sweat away. For a few moments, it feels as if I am flying.

I don’t think a habit like this can last for ever, although some people carry on into their 80s. Fauja Singh ran a marathon at 100. I will be happy if I make it to 70.

And then? I don’t know. Cage fighting looks like fun.