The rowing studio at fancy gym One LDN contains maybe 12 ergs. Or maybe the action is called the erg and the machine is called “rowing machine”. Whatevs. There’s a machine, you strap your feet into it, then you row. You know, like in the 80s.

Laura Hoggins, the trainer taking today’s “engine row” class, has early morning super-pep. “Jump on to the space beside you,” she says. “We’re going to do a quick warm-up.” The first thing I did when I sat on the machine was strap in my feet, subliminally thinking that once I was buckled in nicely, I could go back to sleep. Now I can’t get out. Everyone else is ready to go. I’ve got one foot out, but now I’m stuck. I am very lazy, but hyper-competitive, which is about the worst combination you can devise for an exercise class.

Right. I’ve escaped the machine. I’m standing in my own personal workout area next to the rowing machine, doing squats incredibly fast, which I don’t think is the point. Then the rowing begins.

Like all exercises that involve both arms and legs, the rule is that you use your legs for the power and your arms mainly for finesse. The reality is that you forget you have legs until your arms are about to fall off. The first exercise is four minutes of rowing flat out, aiming to cover 1km. I may not be able to tell my arms from my legs, but I am pretty good at maths. If 200m takes one minute of rowing so hard I can barely stay on the seat, 1km isn’t going to happen, unless everyone else blacks out and I take a couple of minutes’ extra time before I call them an ambulance. Never start thinking about ambulances during a workout class; the next thing you know, all you can think about is prominent public figures who’ve had a stroke while rowing.

I manage 800m, then leap out – I’ve sussed the foot straps – for slower, more dignified squats, press-ups, lat pull-downs and a kind of gravity-assisted row using two straps fixed to the wall (a TRX) with which you pull yourself forwards. I’m doing more maths. If four minutes’ rowing was that hard, and interim weights take five minutes, and the warm-up took no time, and there is nothing else in this room apart from 90s club hits and elegantly rolled sweat towels, how long will it be before they let us leave? And will I still be able to walk?

We spend nine minutes doing interval rowing – 30 seconds on, 30 resting – covering less ground (or, technically, imaginary water) each time, against explicit instructions to cover more. Some more minutes doing more weights; the class is 50 minutes, which vanish like bubbles as soon as you stop obsessively counting them. Afterwards, I feel as if I’ve had an organ transplant from Arnold Schwarzenegger: stupidly powerful and energetic, and ready for massive socio-political jobs for which I am not qualified. Finally, I understand, just a tiny bit and, if only for a few hours, what makes people addicted to exercise.

This week I learned

Rowing burns probably the most energy possible for a low-impact exercise.