I have passed the first test. Often when I go through Waterloo at midnight I fall prey to temptation - a burger, a baguette, a limp pain au chocolat. But not tonight - tonight it is an orange juice, because this is the first day of the rest of my life. I have acquired a personal trainer and together we are strong.

Individually, I am not strong. In fact I am hopeless: overweight, fleshy, unfocused. Journalism is not good for you: peculiar hours, irregular meals, too much booze, and a certain rootlessness. Middle age is not good for you either, and it looms. Before it arrives, and filled with millennial fervour after reading all those glossy new-century, new-you supplements, I decided to undertake some running repairs and a touch of redefinition.

That is how I came to find myself at Matt Roberts at One (translation: Matt Roberts's gym/health club at One Aldwych in London), signing up for a three-times-a-week, no-backsliding-allowed, three-month course that will transform me from zero to hero, from the pits to the Brad Pitts.

The attraction of Roberts's approach, I was assured, is that it is "holistic". The result will be not just a new physical me - desirable though that will be - but a new mental and spiritual me, too. (An annual membership fee of £1,500 and sessions with a trainer that work out at around £45 each is surely not an exorbitant price to pay for this New Beginning.)

To reach the gym, you have to walk through the bar at One Aldwych - a trendy newish hotel, bar and restaurant complex just off the Strand. It is filled with matt-black young things drinking Paraguayan beer, which has a useful dual effect: I simultaneously aspire to their suave, pencil-thin matt-blackness but despise the way an evening is being wasted in idle conversation and booze when, down below, a shiny, shimmering chrome-and-glass nirvana beckons.

My trainer, Sam Jenkins, is terrific. Didn't Madonna once get her trainer to father her child, Sam? But he refuses to mix his work with his life, tempting though that might sometimes be. With me it is it not very tempting. Sam studied physical education and sports science at the University of Evansville in Indiana - he got a place there on a soccer scholarship - and has only recently come back from the US. ("It was great but I missed British culture, the humour and the football." Really?)

The US is of course the birthplace of personal training, and Sam reckons it is about five years ahead of the UK. Whereas here it is still associated with celebrities - witness the recent stories about Anne Diamond and Vanessa Feltz, both of whom have employed trainers to help them lose weight and get their lives back into shape - in the US its appeal is steadily broadening and it is ceasing to be seen as the preserve of the rich and famous.

(Vaguely socially aware note: Clearly, while you do not need to have a regular slot on daytime television to get a personal trainer, you will need a fairly hefty income - I estimate that to do it properly will cost £6,000-£8,000 a year. If you feel that, by examining my psyche in this way, I am selling out to consumer-driven capitalism and should be garrotted with my own ridiculous-looking singlet, all I can do is apologise and recommend you turn immediately to the arts pages while your blood pressure gets back to normal.)

Sam asks me what my objectives are, and I weakly volunteer fitness, thinness, mental strength, all the usual stuff. He warns me not to get too obsessed by weight loss (training can actually increase your weight, but it should at least be better distributed), tests my body fat levels (31%, which, while not catastrophic, is not good), my blood pressure (not bad) and puts me through a not-too-demanding session on the treadmill to check my heart rate during exercise (high but not excessively so and he says it will come down as I exercise over the coming weeks).

So what's the verdict? "You are are a classic middle-aged sedentary male," he says, sounding more brutal than he perhaps intends (so much for my plan to change my life before I hit middle age). "You're not horrendous - there are a lot of people in a worse state - but you're not great either. You are fairly strong and I think there's something to work with." Gee, thanks.

Sam is also impressed by my attitude. "I have a good feeling about your motivation," he says. "You don't seem to be doing it because of peer-group pressure, or spouse pressure, or because everyone else is doing it. You seem to want to do it for yourself and for real. It has to be a lifestyle change, not a vanity thing - you have to want to make a change for life. You seem positive and raring to go, which is a big plus."

So what are we going to do? (Note the "we" - this is a joint commitment; I am doing it for him as much as for myself; guilt, duty, loyalty are major motives for keeping to the plan). He says the long-term goals over the next three months (which in life-changing terms is not very long) are to lose at least two stone, to get rid of 3% of body fat, and to increase energy levels and mental sharpness.

The short-term goals will change from week to week but the first three are simple: eat five pieces of fruit a day, eliminate two snacks or substitute fruit for sandwiches and chocolate (I should have mentioned that the carbohydrate-laden Guardian food trolley parks four times a day about two feet from where I sit), drink two litres of water a day (is that possible without bursting?), cut down on alcohol by around a third (from, say, 20 units a week to 12), and exercise at least three times a week.

I volunteer to cut down on booze even more, but he thinks that might prove counter-productive - "too much of a shock to the system". He says it is better to achieve this grand lifestyle change gradually, rather than try to become a new person (sorry, New Person) overnight. "Reasonable" and "realistic" seem to be his watchwords.

He maps out an initial fitness programme, covering three one-hour sessions a week, each supervised by him. The programme will include 25 minutes of cardiovascular work (cycling, running, rowing) three times a week, resistance work using weights machines twice a week (he says we will move on to free weights, which are harder to control, later), and flexibility training to help with strength and reduce aches. We begin in a few days and, assuming my stamina holds out, I will file fortnightly dispatches from the fitness front. We can win this together.