Nigella Lawson once advanced the idea of using prunes to replace butter in a cake, saying it seemed unlikely, but that’s what they do in California, “where they make it their business to know that sort of thing”. This is what kept coming back to me in Barry’s Bootcamp, an extremely high-voltage exercise class whose composition was created by the original Barry, a gym instructor in Los Angeles. Does it have to be so hard? Yes. It’s from the city of the beautiful people. The principle is that you can fanny around all you like with yoga and aqua aerobics and body conditioning; but the only things that will really make a difference to your fitness – AKA how much of a Love Island body you have – are running and weights. Time-poor and in search of a six-pack? Stick to these.

I did an abs class, in which you move from a bench on the floor with a 5kg weight, doing endless crunches and variations thereof (side crunches, bicycle moves), to the treadmill, in roughly 10-minute intervals. On the treadmill, you are taken through a series of speed intervals, told what pace and incline to set on the machine in short bursts. It’s not as hectoring as you’d expect: perhaps ur-Barry was wont to yell at people about pain and gain, but Sandy, who started the UK branches with his brother and sister-in-law, is quite a reasonable man, saving his motivational blasts for the people who go all the time and really could run 10 miles an hour if they put their minds to it.

Plenty of spin classes deck themselves up like nightclubs, to make you forget how hard you’re working, but this is the only place I’ve been that is red lit, which would make it look like a bordello, except that everyone is sweating and possessed of a demonic determination. Men take their shirts off and women wear sweat bands; surveying this scene in a mirror – which on the treadmills is unavoidable – you all look like you’re running full pelt out of a jungle that is on fire.

Sandy started us on a “jog” that is as fast as I would normally run, working up to a run that is faster than I have ever considered. My thoughts started to deliquesce and slide through my brain like melting cheese. Wow, I look like Jason Bourne. Check out my arms and my serious face. Oh… eight miles an hour? What’s that in kilometres? Oh, no way – this is stupid. Are you mad? Can’t breathe. How long have I been here, a week? Forty-five seconds. Go back to one mile an hour; what does a one look like on this stupid machine? Ah: 1.

It was wrong to bail out, catastrophic for morale; once I took everything back to an incredibly slow walk, two other women did, too, and Sandy’s wild imprecations (to the max!) sounded like dispatches from another time zone. But, sooner or later, the treadmill section is over, your sins are forgotten and you’re back on the floor. There are privacy booths at the back of the room for famous people who want to join but not-join. The Beckhams do it. George Osborne, too, apparently. It doesn’t fill me with any more respect for the man, but I do now see him as slightly more dangerous.

What I learned

Never support your neck with your hands during a crunch. It defeats the object. Better to do a less-good crunch than wreck your neck.

To find a class, go to barrysbootcamp.com