You’ll know British Military Fitness by its trail of shouting; hordes of the red-faced righteous chasing across parks after a tiny, hectoring man in camo. The instructors are probably not that small, they’re just farther away. It is, according to legend, a fantastically fast and effective way to get fit – some combination of the fresh air, the uncompromising style and the comradeship in adversity. It’s exercise as theatre, for people who love exercise and maybe don’t realise how much they love theatre: you engage in the group delusion that you’re actually in the army, and that makes you work harder.

There’s lots of running – slow running at the start, a mile run in the middle, sideways running for warm-up purposes, sprinting at the end, jogging back to the finish. I thought I knew what kind of runner I was, until I did it with other people, when I realised I was rather easy on myself. I was second to last on every turn, until a split second before the end, when the super-competitive woman behind me would come beetling up and trounce me. I was a great motivator for her, put it that way.

My instructor wasn’t a shouter; he was more like one of those teachers who knows you can do better, and you know you can’t, and you don’t want to disappoint him because he seems so nice, but also, what on earth is he on about?

But the bit you’ll notice from afar, the bit that will have put you off if you are at all physically shy – which is to say normal – is the bit where you work in pairs. There is a tacit agreement that women work with women and men with men. When you’re an uneven number, the beta female has to work with the beta male, which is how I found myself doing press-ups into a stranger’s fist. He was lying face down with his fist up and my aim was to dip and touch it with my sternum; I wasn’t up to it and would just brush him with whatever body part landed first, inevitably some breast or other. He’d say motivational things, such as, “Don’t forget to breathe”, while I battled agony and embarrassment, and averted my eyes. It reminded me of something… it reminded me of being in labour. Two minutes felt like an hour.

We did so many sit-ups that my abs went into a catatonic state and I finished using only two dusty vertebrae in my lower back. A long-forgotten song by the 90s indie band Salad started to pound my head, like the beginning of a traumatised state.

We swapped partners; now I had a woman, except a tiny one, about the size of my 10-year-old son, and I had to put my full weight on her thighs and tricep dip. This felt morally wrong and physically appalling.

Then I started to love everyone, which I know was chemical and not real, but what can you do? I love them.

What I learned

From Vanny, who has been going for 10 years: “Only the young progress; for us, it’s just maintenance; maintaining the illusion of youth.” [A pause.] “Maybe if I went four times a week, I would progress.”

• Hair and makeup: Sarah Cherry. Trainers: Saucony Life On The Run Triumph ISO