Like most people who do no exercise, I hold the conflicting views that I am a) pretty fit anyway, just by osmosis. I walk past joggers. I live on the same road as a gym; and b) fitness is for people whose values are not mine. Therefore, it is not at all difficult for me to think that this 5:2 principle, applied to exercise, will most probably not work, but to do it anyway. What do I care? I am already pretty fit. (I am not fit at all.)

"Well, I'll tell you what," David Marshall says, "come in and talk about it. Because, at the end of the day, it's your body, and I'm sure that means more to you than some [stupid] article." (The "stupid" was silent; but I heard it.) Marshall's motto is, "Trust me, I'm the Bodydoctor." He runs a gym and personal training empire called Bodydoctor, but it's the "trust me" that I take most to heart. Why has this never occurred to me, that my health, my irreplaceable vavavoom, might be more important than some [stupid] article? One time, I drank 183 units of alcohol for a Christmas feature. In a week. Not over the entire yule period.

Anyway, I go to meet Marshall. We agree that two sessions a week is not as good as three would be, and yet his method – to train within your boundaries, rather than harass your muscles to the point at which they have to rebuild themselves – is so superior to regular training, he tells me, that two sessions will still do more than normal people doing three. "Seventy per cent of those people are just wasting their energy on huff and puff." I don't really know what this means, but I believe him absolutely. I'm definitely not going to waste my time on huff and puff.

We do my fitness assessment. I'm not fit at all. I am roughly the same meat-to-fat ratio as the cheapest sausage you can imagine. Huh.

"Ninety per cent of people who overeat are actually thirsty," David says, looking at me sideways. It is week two. I have learned a new load of words – eccentric muscle contraction, Bosu balance trainer – and a new load of exercises. Kettlebells are huge now. I don't know if they even existed the last time I was in a gym. There is a lovely manoeuvre called the halo, where you swing a bell around your head and try not to hit yourself with it. It takes so much concentration that you don't notice how much effort it takes, and then, when you wake up the next day, you have pains in areas where previously you didn't realise you had any muscles. At first, I thought I was having a heart attack.

Then I thought, "But I feel fantastic! Wouldn't that be the weirdest thing, to feel this good while having a heart attack? I must be getting really fit."

It can't be proven, can it, that 90% figure? And yet it went into my head, and I bought one of those bottles that filters your water as you drink from it. Then, because they are brightly coloured, the kids wanted one, too, and wham, before we knew what we were doing, we had turned into one of those families who drink water as their main drink.

"How does that feel?" David asks during the third week, as I do a complicated lunge on a powerplate machine (it is the shape of an ancient weighing scale, and it vibrates). I don't know what he wants me to say. "I want to puke?" "I actually take this pain as a reasonable punishment for what I now consider to be the unforgivable narcissism of the whole concept of hobby-fitness?"

"Great! I feel great!"

"I don't believe that," he says, shaking his head. "It doesn't feel great. It's not meant to feel great. It's a means to an end. The end being," he paused, "that you get fitter."

I genuinely trust him. He's the Bodydoctor.

By week four, in the grip of a Nietzschean superiority complex, I realise I have started to enjoy this stuff. I love the training days. I love leaving the gym, full of the vigour of the self-righteous hardbody (it is not really hard, but it's harder than it was; I have enough self-awareness left not to bore you with measurements).

The self-righteouness makes no logical sense. Whom did I help? What did I win? What difference does it make? No one, nothing, none. I will most likely never become an elite athlete, or even enter a triathlon. (David's personal trainers are all triathloners; I filled the air with inane questions – "Which bit do you like the best?" "Which bit do you like the least?" "So cycling is your middle favourite, is that right?" – to distract from my incompetence. They were not distracted, but they were polite.) But I feel alive!

If you want to know whether the two-day-a-week principle works, it does; even that frequency is habit-forming. It creates other healthful habits: I started making ridiculous snacks for myself – dainty asparagus spears on rye bread – as if trying to tempt into appetite an invalid from the royal family. The sense of being rationed makes you work harder, and for longer, and makes it all feel like less of a chore.

On the matter of exercise itself, I am ambivalent. I think it makes me more rightwing. But I feel alive! But I'm a dick. But I feel 22! But… you see what I mean? Ambivalent.

How to make it work

• Resistance training is the most important, especially when you're doing it only twice a week. Work with weights that exhaust you, doing between 20 and 25 repetitions.

• Exercise early in the day.

• Keep hydrated – eight glasses of water a day. Ninety per cent of people who overeat are, in fact, thirsty.

• Eat before and after you exercise: complex carbohydrates beforehand and protein within an hour and a half afterwards.

• To order a book and DVD for £25 (RRP £50) visit bodydoctor.com/guardian.