You know how occasionally someone you know will suddenly do something so wildly uncharacteristic, you begin to question whether you ever really knew them at all? You've known Jane for 15 years. She's always been a vegetarian. And now she's married a human being made of meat. You're confounded and slightly hurt. Who exactly was this "Jane" you spent so much time with? What other surprises might be lurking within the Jane-shaped shell you once called a friend? Where was she on the night of the fifth? Is that her real leg? Who is Keyser Söze? Etc etc.

Still, if it's slightly creepy when a friend behaves atypically, it's borderline terrifying when the person behaving out of character is wearing your shoes and your haircut and looks like you and is you. Take me for instance. For years, I thought I knew vaguely who I was, and the kind of things I liked. And one thing I'd definitely class myself as is "un-sporty". I've never had a gym membership and have always been profoundly suspicious of anyone who willingly does anything more physically demanding than wiping their arse. So imagine my shock, in recent weeks, to find myself running around a local park. Not once, not while being chased in a waking nightmare, but voluntarily and often.

I confess: I have become a runner. I go running. I run. Like a runner. Which is what I have become. A running runner. Forgive me. Oh Christ. Forgive me.

It started innocuously, not to mention geekily. I stumbled across an app. An app designed to encourage couch potatoes to "get into" running by easing them in at a pace so non-threatening you'd have to be physically glued to the sofa to be daunted by it. Here's how it works: you pop a pair of headphones in and put some music on. Then you start the app. It fades the music down for a moment and tells you to stroll around for about 90 seconds. Once that time limit's up, it interrupts again and politely asks you to run for 60 seconds. Sixty seconds, no longer. Then you walk for 90 seconds again. And so on. It's literally a walk in the park. And before you know it, the app's voice – a slightly patronising female whose accent hovers somewhere between Devon and Melbourne – is saying well done, that's enough for today, you can go home now, and incidentally you're wonderful. You repeat this three times a week; each time, it incrementally lengthens the run and shortens the walk. After nine weeks, to your own astonishment, you're running uninterrupted for 30 minutes.

I always hated healthy outgoing types. Really despised them. And when they smugged on about how physical exercise gave them an endorphin rush, I felt like coughing blood in their eyes. Now, to my dismay, to my disgust, I discover they were right. If I don't get to run, I become irritable, like a constipated bear that can't find the woods. I have to get out there. And I run for longer: I'm up to an hour at a time now, sometimes more.

I remember the psychological barrier I had to pass through when I bought my first pack of cigarettes. I'd cadged here, dabbled there, mainly at night, over a drink, until finally one day, I had to face facts: it was the middle of the afternoon, and I was gasping. I popped into a newsagent's and bought my inaugural pack of Marlboros with a burning sense of shame.

I don't smoke any more, but I felt that shame again a few months ago, when I finally snapped and bought a decent pair of running shoes to replace the crappy trainers I'd been using. Once that dam was broken, I bought some wanky running shorts. Not one pair – but several. I even bought a preposterous sports top made of some kind of cybernetic superskin designed to slurp sweat off your back and email it to a parched section of the developing world. It's a fabric with its own trademarked name and diagram, squarely designed to appeal to the kind of person I hate, and I own it. I can scarcely bear to look at myself in the mirror.

This is how low I've sunk: I went on holiday recently, all the way to Australia, and on the way there we stopped in Singapore for a night and I … I can scarcely type this … I used the hotel gym. At 6.30am. God help me I ran on a treadmill at 6.30am. With other people in the room. And then I went on a cross-trainer. In full view of everyone. It feels good to admit it. It feels cleansing, somehow. And that was the first day of the holiday. I ran as often as I could after that. And then flew home and ran some more.

Running, exercising, using gymnasiums … it's a betrayal of everything I stand for. I hope it's some kind of temporary life crisis. Or a complete mental breakdown from which I'll eventually recover. Otherwise I'm going to have to start physically beating myself up. And even then, even as my own fists swoop towards my self-hating face, I'll be secretly anticipating the endorphin rush of all that extra exercise. Doomed. Doomed.