I might as well tell you, seeing as I've bragged about it to everyone else, that I've taken up Bikram yoga. It's a variant of the old-lady yoga that everyone professes to be good at, but is done in a room heated to 100F. Walking into that room is like being walloped across the face with an old sock. An old sock that eight people have farted into, has been buried in peat, dug up, farted into again and then filled with sick. I have never in my life encountered such a smell.

"Please be careful of our new carpet!" shouted orange signs around the room. "If you're lucky enough to sweat so much you leave a dark mark, please use three mats and an extra towel." The carpet did not look new, so I had wrongly assumed its secretions were to blame for the acrid funk but, in fact, the smell was a permanent miasma, a steady state of stink that was entering my lungs even as I tried to gag it back out again. It is terrible, the realisation that a stranger's vapourised bum sweat has been on your tongue, in your nose and down your throat – and that you paid for the privilege of it getting there.

"No pain, no gain," I thought. I don't really like exercise, if I'm honest – I don't like either sweating or the sensation of doing exercise – but I chose Bikram because it seemed the most torturous, self-immolating way to lose weight. You want to feel like it's working, right? Plus the class was quite close to my house.

"If you feel dizzy at any point," said the receptionist, "just sit down and breathe through your nose." I nodded and rolled my eyes like a pro. I'm quite a flexible person, both literally and metaphorically; I'm also quite a sweaty person, and I spent my teenage years fainting quite a lot. So I was singularly equipped to deal with the stringencies of my first 90-minute session of Bikram.

It wasn't until I entered the room that I realised quite how long 90 minutes is. "It's nearly half of Gone With the Wind," I thought. "I could get from antebellum to Reconstruction in that time. It's slightly longer than a Disney film, actually. The Little Mermaid will have legs long before this class is over."

I lay down. I was instantly too hot. I shimmied a bit and got even hotter. I realised too that there was a lot of flesh in the room. There were men in Speedos, women in leotards and a woman right at the very front in a pair of tiny pants. She couldn't take her eyes off herself in the mirror that spanned one wall of the studio. I couldn't blame her: she had the sort of thighs that don't change circumference from knee to hip joint.

In the spirit of the thing, I took my T-shirt off and sat in my shorts and my bra. Nobody seemed to mind. It was like being in a club full of people who were absolutely, completely and to a one, off their heads on drugs. They all had staring eyes and no interest in anyone else. We had all gone into robotic survival mode.

We started with some deep breathing. "A cinch!" I thought, wondering why my shoulders didn't bend like everyone else's. And then: "HAAARRGGGHHH!" Everyone began rasping and snarling like Hollywood zombies. It was terrifying, but apparently you're encouraged to make that noise when breathing out, as part of the detoxification process. "The heat's made you skittish, Harry," I thought. "They're not really zombies."

It was at this point, as we all moved damply into the half-moon pose, that I needed to be violently sick. Swallowing hard, I continued to point at the ceiling and snarl like the undead. Soon, the edges of my vision went all sparkly. I crouched on the ground and wondered what the hell I was doing in a room that smelt of cadavers, in my underwear, surrounded by fitness freaks, trying not to be sick on to a sweaty new carpet. It was too much. I scuttled out.

"Are you OK?" the instructor called after me. "Oh yeah, I'm good," I gargled through a mouthful of sick. Outside, I bumped into a gentle guru who tried to talk me into going back in. I almost cried, through eyes that were no longer working. "Sometimes it does feel a bit much," he soothed, "but that's just because you're detoxing."

"No, it's because it's really hot." I countered, trying to force my T-shirt back on, having realised I was practically in the buff. It got caught in all the sweat on my neck and became a knotted flannel round my throat. "I'm not going back in there," I said as I shouldered past him, "but see you on Thursday!" I added as an afterthought. Yes, I'm going back for more. I might not manage 90 minutes, but I hope to stay as long as it takes Scarlett to tell Ashley that she loves him. That's only about 20 minutes in, isn't it?