It's over. My three months is up. I'm on my own. You will first want the facts. I've lost a stone; my body fat is down from 32% to 25%; my resting heart rate is down from 86 to 68 beats per minute; my blood pressure is the same - high, but not worryingly so. That is good progress, but by no means the end of the story. Ideally, I ought to lose another 7-9kg (15-20lb), reduce my body fat to around 20%, and get my resting heart rate down to the low 60s.

Sam, my personal trainer, has kindly sent me on my way with an action plan. This will mean working out twice at weekends, combining half an hour of cardiovascular training with half an hour of weights; once in the week, with 25 minutes of cardiovascular and 20 minutes of upper-body work; and running twice a week for 30-40 minutes each time.

He emphasises that what counts is the intensity at which you train. You have to push yourself; get your heart rate up to, in my case, 150-165 beats per minute (this figure will vary according to age); introduce "speedplay" into the running, injecting short bursts to raise the heart rate. Rest assured that I will do it all - scout's honour.

So, some conclusions. It has certainly been worthwhile and I've gone sufficiently far down the road to want to carry on with the journey. I've really enjoyed locating, if not quite uncovering, my stomach muscles. I realise now that booze is, mentally and physically, very bad news: I'm down from 20-plus units a week at the start to maybe five a week now. Drink should be an occasional pleasure, not a daily standby. It's a treat, not part of the routine for getting through the day.

I haven't quite worked out diet yet. I have muesli for breakfast, eat much more fruit, spread little or no butter on toast, and eat smaller, more sensible meals through the day, but I still find it difficult to kick biscuits, sugar, muffins and, worst of all, cheese. (Although, if cheese is so bad for you, why do you never see fat mice?) The real problem is that I don't have the imagination to plan a diet, and so end up eating endless tubs of tuna pasta, which isn't even that healthy.

What I don't do any more is have any really large meals: I now never feel bloated after eating. One course rather than three; water rather than wine; say no to rolls; avoid desserts. It isn't puritanism, it's pragmatism - your body quickly gets used to the new regime, and sitting down to a three-course meal with half a bottle of wine would now be anathema.

I still don't drink enough water. A litre and a half a day really is a lot; you spend your whole day wondering where the next bottle of Evian is going to come from, which drives you crazy after a while. (Consumer note: the small bottles are wildly overpriced, so get organised and buy a supply of large bottles from a supermarket. [Hypocrisy note: I never do this, but it doesn't stop me passing on the advice.])

Motive: I've tried to convince myself that my urge to train, to get thinner and fitter, hasn't been driven by narcissism. No doubt that plays a part, but I don't think narcissism would be enough to keep you going. The real pay-off is control, physical and mental. We eat and drink too much, get fat and unfit, out of boredom, laziness, indiscipline and perhaps a sense of failure. Reversing the process is all about empowerment.

We complicate life to a ridiculous degree; the gym is a place where all the frustrations are stripped away and life is reduced to something simple and linear. Exercise is about your psyche rather than your body, so perhaps it is a form of spiritual narcissism. (Note to publishers: I am very happy to flesh this out in a short, money-spinning self-help book called something like Reclaiming the Self.)

Sam's point was always "How do you feel?" not "How do you look?" Lose weight at your own speed; don't be a slave to the scales; don't diet just for the sake of it. Your well-being and performance in training are the best guides to how things are going. I don't feel I've got much stronger over the three months - my left side is still much weaker than my right, for example. What has improved considerably, thanks to all the cardiovascular work and my more efficient heart, is my running: I now feel in control when I run, and that's when it becomes fun.

Now I must leave Sam: my friend, guide and taskmaster for the past three months. A colleague suggested that I write a follow-up series called "Piling it all back on" - a fortnightly exploration of decadence that he thinks would be far more popular than my advocacy of the ascetic. But that would be an insult to Sam. He says that if I fall by the wayside now, he will have failed. As we said at the beginning, this change is for life.

The intention henceforth is to eat well, drink plenty of water, drink less tea (and coffee only occasionally), eat lots of fruit (five portions a day, remember), drink alcohol for occasional pleasure and not as a regular pick-me-up, go to the gym three times a week, run at least twice a week, keep up the boxercise (but avoid fights), and use the new-found fitness to be more mentally focused (physical health ought to have a spiritual purpose). I'll stay in touch with Sam, too. Next season he is playing for a football club close to where I live and is going to come for the occasional (light) pre-match lunch. So, you see, it's a happy ending. Or do I mean beginning?

• Stephen Moss would like to thank all the staff at Matt Roberts at One, 1 Aldwych, London WC2 (tel: 020-7300 0600), where he worked out for three months.