There are many fitness pursuits that claim to be “the single best thing you can do”. I don’t normally have much truck with them, since that is usually code for “incredibly hard”. However, indoor rowing has plenty of disciples I trust, so I turn up to a class at 7am. You cannot overstate the seriousness of people who have built that into their routine. You also should not underestimate their nous. They are here because there is no better use of 45 minutes in the pursuit of health. So let’s begin.

Indoor rowing is strangely engrossing when you have a target: for our class, that means covering 800 metres in four minutes, or 1,000 metres if you are fit. Nobody badgers you about your technique, nobody tries to motivate you with “you can do it” boosterism; there you are, numbers in front of you, arms and legs, done. It is fiendishly hard, but time-limited. The interior dialogue is so short at four minutes (“I can definitely do this! No, wait, I can’t, it’s really unpleasant! Actually, you know what, maybe I can?”) that your demon self doesn’t have time to win.

At the end of that four minutes, however, you don’t get to reward yourself with a nice lie down. Instead, the class is run like circuits: four minutes on the machine, four minutes in the gym. Since this is a rowing-focused class, the gym moves are also beastly. It starts with throwing a medicine ball up a wall, quite high, then catching it. The ball is extraordinarily heavy. I have to swap to one half its weight and I still find it bafflingly difficult.

Then there are the fashionable burpees and kettle bells. The next round features “suicide press-ups” and jumping jacks. I am surprised to find myself relieved when the rowing starts again.

But the time goes like mad with this kind of caper: factor in a warm-up, a cool-down and a welcome one-minute break between each segment, and you have to do each thing – rowing, then circuits – only three times. Then you are done.

They have got this stuff down to a fine art, and not just at Metabolic in London, where I take this class, but at F45, Barry’s Bootcamp and the whole slew of high-impact, fast-churn gyms that have sprung up for maximum efficiency, mainly in affluent areas and very chi-chi environments. They know exactly what to do to generate energy while forestalling boredom, and they get out of you the most they possibly can. If you can do it first thing – which, considering that so many of the classes are early in the morning, you get the impression you should – you feel terrific for the rest of the day. I didn’t conduct an extensive field study, but everyone there looked great. I don’t mean “thin”; I mean stone-cold great – fit, energetic, ruddy, toned, determined. It might just be the single best thing you can do.

What I learned

It’s easier, and better improves your fitness, if your kettle bells are slightly lighter, not heavier, than you are comfortable with