Twice a year the British confess their sins of greed and sloth and vow to mend their ways. The first time is straight after Christmas, when most of us look like the turkeys we've eaten. The second time is about now, when the holiday season makes men flex their pecs, only to discover they don't have any, and we girls dust down the bikini to find that it only fits one boob and half a bottom. At such moments the cult of the body becomes a new religion. We adopt dietary laws that read like purgations - take one small chicken wing, a skinned tomato, half a dozen melon seeds and a drop of balsamic vinegar and we swear never to drink again until we reach Marbella.

To show our poor bodies how serious we are in the subjugation of the flesh, we join a gym and promise to go there three times a week. Unlike most churches the gym is open all day and half the night, so whenever you want to wrestle with the Flab Demon, you can.

Last week Mintel, the nation's trend-watcher, announced that gym membership is soaring. In the past 10 years membership has doubled, and the upward curve shows no signs of tiring.

Is this a good thing, or needless punishment? And what happens to all those people who pay the money for membership - often £1,000 and more? Do they stick it out and go on to become healthier, happier, better, stronger, or do they fall at the first feeding frenzy?

Well, at the risk of sounding like reality TV, I will come forward and make a personal confession. When I turned 40, which now seems as long ago as when dinosaurs ruled the earth, I realised that things were going horribly wrong. I weighed eight and a half stone, which is about a stone too heavy for a five-footer, and I reckoned that if my boobs got any lower, I would have to buy them their own pair of shoes.

The arms were getting like Elizabeth Taylor's without a bank raid of diamonds to compensate, and my tummy made me feel like one of the seven dwarves. No, it made me feel like all of them.

So much for the aesthetics. My sit-down job has never suited me because I hate sitting down, and as the metabolism slows as we get older, sitting down turns your lower body into a stone gargoyle. You lose mobility, flexibility and strength. So I read Dylan Thomas - Do not go gentle into that good night - and I set off to join a gym.

This was not the first time. I had been extremely fit in my 20s, and I even wrote a fitness book, straight after Oranges Are Not The Only Fruit. Thankfully, this is out of print, and what a good thing it is that Oranges was a success, otherwise I would have ended up setting up a studio in Covent Garden and training the stars - yes, that was my plan. So, if you'll forgive me for that brief excursion into another life, my point is that I knew what was happening to my body, and that I could fix it.

But, you may say, isn't it just a pathetic attempt at denying the ageing process? Yes, of course it is, just like going to the dentist, wearing make-up, or dying your hair. Now you could go grey and let all your teeth fall out, but you're not going to are you? So why drag round a body that makes you miserable?

I am not a body fascist, and I don't believe that small is beautiful or that you can never be too thin, or that Geri Halliwell has the answer. What I do believe is that being healthy and fit makes life more fun and that for women in particular, a body you love is better than one you ignore.

My experience in gyms over the years, and conversations with my trainer, reveals some facts not obvious in the Mintel report. First, the drop-out rate is high for both sexes. This is not an easy statistic to monitor, because once you join a gym, even if you never go, or if you visit infrequently, your name is on the books boosting the takings and the tally.

Women more than men appear to be serial gym joiners; a year here, a year there, nothing quite working, but still the desperate attempt to try. "This is my fifth time," is pretty common.

Sadly, gym-joining seems to be the new dieting; a sudden splurge of effort, poor results, give up, get fatter, join a gym again.

My trainer tells me that it is difficult to motivate women to work hard. We don't like to get red and sweaty, and we have a terror of turning into Popeye, with spinach-fed muscles bursting out of our little black dress. I see this terror at work every time I go to the gym, where women built like cavemen are swinging around weights the size of bean cans - and simultaneously worrying that their body isn't changing shape but that they don't want to build muscle.

Well, I do 15kg on my bicep curls and, just as a comparison, I weigh 47kg, but I don't look like Lisa Lyons (pity) or even Madonna (more of a pity). What I do have is toned arms, with a visible shade of muscle when they move. Sure, if I flex them, all is revealed, but women don't go round bending over in bicep flexes, only strange men do that after washing the car and before sex.

Fact: it is hard for women to build visible muscle. Fact: it is easy for a woman of any age to tone her body and achieve satisfying results, providing that you keep going and you work hard.

But it's so boring! I hear you cry. Yes, it's true, the gym is not the place for intellectual fulfilment or mental stimulation. That comes later, because what the gym does do is make you more alert and less tired.

Can't I just go running? Yes, if you will do it even when it is raining or freezing, and if you will do it for an hour three times a week.

A gym is a very efficient way of getting fit and staying fit, providing that you use it properly. You need cardio-vascular training and strength training to achieve all-body health, and the gym can provide that.

Make it better for yourself by having a sponge-bag filled with goodies so that the shower is a treat. I have so many expensive lotions and potions in mine that I should insure it. It is part of the ritual of giving time to my body and not just my head and for me, because I think all the time it is a great relief, almost a meditation. It is the only time my mind stops, and I am glad of it.

Don't expect an overnight transformation. I lost half a stone in three months, but then it took me nearly a year to reach seven and a half stone, and that is where I have stayed for the past two years without any extra effort. Muscle is heavier than fat, so the scales won't tell you everything, and remember that muscle is active tissue, unlike fat, which is passive tissue. Muscle will munch up your calories for you, while fat just stores them on your bum for later.

I go to the gym four times a week - two days on, day off, two days on, weekend off - so it is hardly arduous. When I'm on holiday or away, I take a blow-up ball with me, go running and then do my strength exercises and abs on the ball. A trainer can show you how. And if you do join, a trainer for the first three months can keep you motivated, as well as making sure that your work-out is right for you.

If you have had children, you can still ping everything back into place, but professional help here will really make a difference. The film-maker Beeban Kidron re-shaped her body so successfully on Pilates (four times a week) that I was determined not to fail, (yes I am competitive), but the big inspiration was Ruth Rendell.

Ruth started working out at 40, and she has never stopped. Now she is in her 70s and she can run up the street, touch her toes and wear jeans like an advert for Armani.

In the end it's about quality of life. The gym isn't a religion or a drug and it won't work miracles. What it can do is help you to feel better in every way, and that includes self-esteem. Besides, you can give up the twice-yearly flagellation and get out the bikini and head for the beach.

That's got to be worth 150 hours of sweat hasn't it?