John Walsh: There's a six-pack here, I know it

For years his muscles lay wasted. But then John Walsh joined the ranks of the stars and got a personal trainer. Now he has found, even in his creaking frame, a rippling body waiting to get out

It is 9.15am and I am horizontal in the living room. No, I am not lying in hog-like slumber after some ill-advised night on the tiles. On the contrary, I am wide awake, fantastically alert, sweating profusely with physical effort, wholly focused on the cord carpet and the pain in my arms as I dip rhythmically towards a rather disgusting coffee stain beneath my nose. I am aware of cutting a slightly ridiculous figure – not because I'm doing press-ups, but because I'm doing them while lying on a huge blue rubber ball the size of a Space Hopper. My groin is resting on the apex of this bouncy sphere, while my manly torso plunges up and down, and painful gasping noises escape my lips. To an innocent observer, I must somewhat resemble a man trying to have sexual congress with a pumpkin.

There is an observer in the room, as it happens, but he is no innocent. He is the man responsible for introducing the ball, the press-ups, the sweat and the whole exercise regimen into my life five months ago. Michael White is a personal trainer. He is, in a sense, "my" personal trainer in that I can throw this proprietorial usage around ("No more Scotch for me, thanks – I've got my personal trainer coming in the morning") in a fake-grand way as if I were talking about my butler or chauffeur, though strictly speaking I am forced to share Michael with a couple of dozen other clients. But there's something very gratifying about name-dropping that you've got a PT. It puts you suddenly in the same class as Madonna, Angelina Jolie, Ralph Fiennes, Pierce Brosnan and, er, Vanessa Feltz (though you don't of course have to have an affair with him, before, during or after your quadrilateral stretches).

It started in February when Michael dropped a card advertising his Home Health and Fitness operation through the front door. My consort Carolyn, in one of those unilateral moments that chaps learn to dread, decided that it was time we got less broken-winded and stoop-shouldered and signed him up. After a preliminary chat of mortifying embarrassment ("So, let's see – apart from occasionally walking to the Superfoods convenience store, the only exercise you take is...?") we embarked together on a punishing regimen. Michael would appear like Santa Claus, carrying an exercise bike, boxing gloves, weights, lengths of rubber hose and the enormous Space Hopper. Standing smiling on the mat, he resembled a benign, 22-year-old professional torturer in his Beckham No 1 haircut. All personal trainers (and I've met a few) are apparently required by law to have their hair cropped or shaven like Beckham or that Jason bloke from Snatch.

Regrettably, after six weeks of synchronised lunging, abdominal raising and buttock-clenching, Carolyn dropped out. It was all too macho for her, the boxing, the cycling, the 48 sit-ups. No offence and all that, but she was going to find a lady trainer and co-opt a couple of girlfriends in a sort of high-energy knitting circle. Pathetic.

I, by this time, had made the odd discovery that I enjoyed this new physicality. For a couple of years I'd felt an uncomfortable sensation of having hundreds of muscles lying so wholly unused that they were quietly packing up and expiring. I had taken to nodding off at 4.05 in the afternoon. My body felt as though it needed a big, first-thing-in-the-morning stretch; one involving a rack and a couple of teams of carthorses straining in opposite directions. But after a couple of sessions with Michael, I was not just feeling better, I was coursing with energy and adrenalin.

On sunny mornings, the fitness session starts with a run in nearby Belair Park. Michael and I jog companionably round two whole circuits of the greensward, past the swings, the basset hounds and their lady walkers, the reeking stagnant stream, the cricket pitch, the posh restaurant... He laughs at my bronchial spluttering, and we talk about this and that, in between gasps (mine) and encouraging little murmurs (his) of "Gently bently, John – don't overdo it". He enquires sarcastically how many parties, bottles of wine, ciggies and late nights I have been through lately, and goes "Tsk" with disapproval like a scoutmaster. He is all of 22. I learn about his hometown (Rye), his long-suffering girlfriend Claire and her spectacular chicken-with-pasta-and-broccoli suppers, about the dents in his white van, his punishing weekend cricket matches, football matches and rugby matches, his pulled hamstring, the contusion on his forehead (stray ball), his trashed knee, spatchcocked groin, sore arm and buggered Achilles' tendon. It's a dangerous business, being as fit as him.

Now you might wonder why a chap should pay for another chap to run round a park with him. Why, you ask, can I not run round the park by myself? I know the way. I have the clothing and trainers. I could do it myself. But I just know I wouldn't. I've tried, but I've always found more pressing things to do. I've tried jogging across the common, but the far horizon never seems to get any closer. Shame-faced though I am to say it, I need Michael to act as my conscience – to say: "You must do this, you know it's good for you."

Home again, we do some boxing in the living room. Of all this exercising stuff, the boxing was the biggest surprise. I've never done it before in my life. In my teens and twenties I metamorphosed from herbivorous post-Woodstock hippy to effete would-be-Tynan poseur complete with liquorice-paper cheroots. Neither image suggested that a closet pugilist was longing to be let out. Now, twice a week, I put on the red gloves and thump the hell out of two padded shields held up by my quaking trainer. You get into this rhythm, you see, this right, left, right, left, unstoppable, accelerating, thwacking frenzy. When you're doing crossed uppercuts, your elbows poke sideways in a ludicrous swaying motion, like one of those Olympic walkers. After five minutes, you feel like the young Jake LaMotta, however much you may still resemble the older version.

Then, with shoulder muscles twanging like a painful guitar, we do the shag-the-pumpkin press-ups, and exercise the arms with bicep curls, using a length of elastic and two handles. You can employ weights, for the authentic "I'm down the gym" tough-guy feel, but they only work with some people. It's very irritating to lie on your back doing arm stretches with two-kilogram barbells and have one fall on your chin because it's slightly too heavy for you. Yanking a stretchy rubber gizmo, with elbows tucked in to your sides, is rather satisfyingly like riding a horse or (the more elaborate version) rowing a very large boat. You can, genuinely, feel muscles in your shoulders being awakened, muscles which had never been shown a good time before.

You do a dozen of each activity, counted down by Michael with cries of encouragement and approval ("Nearly there – last three now – and relax"), take a break for 10 seconds and do it again. "If I told you that you'd be doing 192 press-ups and sit-ups, you'd never get started," said Michael, revealing his cunning strategy. "But if you do it in little groups of 12, it seems quite bearable, doesn't it?"

Then you lie down for the extended-leg stretches, and the final serious abdominal sit-ups. They're difficult, and rather painful, and it's hard to breathe while you're straining forward towards your crooked knee while simultaneously trying not to fart, and they leave you feeling internally bruised – but by the end you're able, miraculously, to feel three double-sets of muscles groaning and complaining inside you. A six-pack! My God, I've got one after all. I always knew I had it in me...

Michael and I do not go a bundle on all the jargon about "lats" and "pecs" and trapezium muscles. The only mention of musculature is "Now for the quads", which means we grab hold of one calf and stand awkwardly like two flamingoes trying to communicate at a party. And then it's all over, the running, the gasping, the boxing, the swearing, the heaving, the bobbing and weaving, the lunging and straining. You are invigorated, drenched in sweat and shagged out, all at once. You're glad it's over and you sign up for the next 10 sessions. If this is masochism, at least it's a kind that makes you feel better the next day.

Many people assume that a personal trainer is a kind of executive accessory for the idle rich, an indulgence for people too indolent to make it down the gym. What rubbish. Over the years, I have spent so much cash joining gyms, so much energy doing knackering self-devised workouts, and so much time devising displacement activities to ensure I never go back to the gym again, that the PT option is a relief. Rather than you forcing yourself to go to a gym, the one-man gym comes to you. He knows, far better than you, precisely which kinds of exercise will tone up which aching and saggy bits of your rotting corpse, and he does the bending and stretching alongside you so you don't feel solitary or foolish or stressed-out.

And while a gym will set you back, say, £2,000 and you'll use it – what, 10 times? – a personal trainer's regular attentions, twice a week all year round (that's a hundred fitness sessions) works out at £3,000 a year for 10 times the results. Plus you get the company of someone who knows precisely why and where you feel bad inside your creaking, middle-aged bones and skin. And who will tell you all about it, in kindly tones, over the weeks and months, as you go thundering past the startled moorhens and stagnant waters of Belair Park.

HomeHealth Fitness can be contacted on 07980 545324