It would not be an easy challenge. I played a lot of sport at university, but have since let myself go. Mainly, I have let myself go to the pub. This, then, is the diary of my gruelling attempt to get a "beach body" in four weeks.

Met Greg Chapman, my personal trainer from Virgin Active gyms, for an initial assessment. Greg is originally from Adelaide, where he played professional Aussie-rules football, a sport so violent that he was knocked out nine times during his career.

He was born on a Greek island, and his middle name is Adonis, the god of six-packs.

Greg weighs me (I am 74kg), frowns and declares that I should aim to lose 6kg. We start with a five-minute "warm-up" at 8kph on the treadmill. "That's nothing," says Greg. "Now I'm going to beast you."

"Beasting" involves running for a minute at 15kph, followed by a short recovery period, followed by another minute at 15kph. This kind of interval training is, apparently, nine times more effective for weight loss than trudging along at the same slow jogging speed. It's also a lot less boring.

Twenty exhausting minutes later, we move to the weights area, where the mirrors must be concave because I look like a heffalump. The best way to lose weight, according to Greg, is a combination of running and cardiovascular circuit training - that is, doing unnatural things with weights while permanently out of breath. It also helps add a bit of definition.

After 40 minutes of lunging, squatting, press-upping, sit-upping and bicep-curling, my decidedly undefined body is ready for a very long shower.

"Try to do 100 sit-ups and press-ups at home every day," says Greg, as a friendly parting shot.

Beginning to get the hang of this - at least, I'm no longer in danger of falling off the back of the treadmill. Greg, sadist that he is, has noticed this and started increasing the speed. Yesterday, we went outside and did punishing shuttle runs in the park.

I've always been a little sceptical about the value of personal trainers, believing them the expensive indulgence of bored, sexually predatory housewives incapable of counting to 10 by themselves.

But Greg is very good indeed. Part of the appeal is that he pushes me 20 per cent further than I'd be happy to go myself. He is also so touchingly enthusiastic that I don't want to let him down, despite his increasingly bizarre exhortations.

"You're a soldier, mate," he said this morning, "leading your troops up the hill at Gallipoli."

The buddie movie continued this afternoon with a trip to the supermarket to sort out my diet which, apparently, is "full of crap". Greg has a point: there's obviously no point exercising for an hour every day if I put it all back on again. So beer is right off the menu, as is pasta, bread and puddings.

It's out with chips, in with fish, steak and chicken stir-fry. If I have to have a drink, I should wait until the weekend and limit myself to a glass of champagne - on expenses, ideally - or vodka and soda water, the drink of supermodels.

Survived the weekend without succumbing to temptation, although there was an emasculating moment when a waitress gave my girlfriend the chicken salad and me the burger and I had to point out the confusion.

I've also mastered the art of ordering skinny lattes in public without embarrassment, and the fridge at home is filled with spinach and fruit, turning me into a curious mixture of Popeye and Bananaman.

I've even developed a strange, pregnant woman-type taste for dry porridge oats. Surely it won't be long until I look like the buff Scotsman wielding a hammer on the packet.

End of the first week and the combination of low-carb diet, running and circuit training has helped me lose a triumphant 2.5kg.

However, I am increasingly baffled by the sheer number of ways in which you can torture your body. For instance, Greg reveals his favourite abdominal workout is the "bicycle crunch", which involves lying on your back and touching elbows to opposing knees 40 times.

It requires a degree of co-ordination I currently lack. Dissolving into hysterics, we rechristen it the "dead ant" exercise.

I can now do 20 press-ups and 100 sit-ups without stopping - a five-fold increase on day one. Muscles appear to be developing from nowhere.

Back in the running groove - I'm doing intervals at 20kph now - and I've turned into one of those horribly competitive gym types who checks how fast the person next to them is going and then tries to beat them.

At least the TV screens make it easier to switch off from the pain, especially when The Apprentice is on and I can give it 110 per cent, like the candidates.

Another weighing session and I'm down to 70kg, mainly due to the circuit training, which has turned particularly evil. Its genius is in isolating body parts. Just as you think your arms are going to drop off, you switch to sit-ups.

Then when your stomach is screaming in pain, you move to leg-burning squats. And then it's back to arms all over again. My least favourite is "burpees" - a press-up followed by a leap in the air - which combines everything at once.

This afternoon, I was "rewarded" with a boxing session. "Chicks love this," said Greg, as I jabbed and upper-cut into the pads. "They pretend they're hitting their exes."

Fell off the wagon last night at a friend's birthday do and resolved not to drink again until the end of the month. It takes 10 minutes on a treadmill to burn off one gin and slimline tonic.

Final week of the challenge and I'm down to 68.5kg and attempting to add a bit of definition. That means lifting heavier weights with fewer repetitions than before. It also means having chocolate-flavoured protein shakes (sounds like a treat, but actually one of the most disgusting things ever invented) to help with muscle recovery. I'm already having dreams about exactly what I'll eat and drink when I finish.

The end! Over the course of four weeks, I've spent almost 30 hours in the gym, run 75km, lifted an accumulated weight of several tonnes, sat up 3,000 times, pressed up 1,250 times and lost almost a stone. The annoying phrase "no pain, no gain" has acquired new meaning.

While I've spent much of the month exhausted and hungry, there were definite positive side-effects. I might be 23/25ths of the man I used to be, but I feel mentally much sharper. Even though my trousers no longer fit, I now have the confidence to wear T-shirts in public again.

"Tell Greg I love him," said my girlfriend, whom I've just about managed to keep away from him.

Inevitably, I celebrated by finishing with something of a binge. But instead of "doing a Prescott", I surprised myself by going for a voluntary run in the park the next day. The only challenge now will be staying in the same shape until the sun does come out properly again - some time in early September.