The path to a portly tummy is paved with good intentions: empty promises of pre-work yoga, lunchtime running, evening gym sessions. Britain is riddled with twice-worn swimming costumes, barely broken-in trainers and sports bras greying glumly at the bottom of the knicker drawer. We are a nation united by its abandoned sportswear, common ambition to make it to the gym, and a collective excuse: "I didn't have time."

If the government could raise a tax on this phrase, it would probably have enough cash to buy every single adult their own personal trainer.

Meanwhile, we try and make our own amends. Really, who needs shouty people in Lycra when there are desk-squats, stair-climbs and walking to the shop to fit into a busy morning?

Don't pretend you don't understand what I'm on about. All those attempts to make up for our gym slackness: running up escalators, doing a bit of Pilates-lite while lying in front of the telly, two hours enthusiastic disco dancing at that wedding last weekend (which got especially high-impact when Wham! came on).

Everybody is at it - gym-shirkers on a mission to rack up bonus exercise points in the fight against heart disease, cancer and cream cakes.

The problem is, none of us really knows if all this covert exercise action is having any effect. We have no ammunition when our binge exercising colleagues belittle our efforts and scoff at our brags of extra-curricular ambling. Surely, every little helps?

Asker Jeukendrup, professor of exercise metabolism at Birmingham University, has some cheering news. "It all depends on what your goal is," he says. "To get the health benefits, there is no need to do sport or go to the gym, just being physically active is enough."

But, it transpires, there's active, and there's active. "Millions of years ago," says Jeukendrup, "humans' physical activity levels were really high. Now we think activity, like the Tour de France, is totally extreme. But then, it would have been normal."

So while the average office worker burns about 2,300 calories a day, our ancestors would have been using around 5,500. You are unlikely to make up the difference by getting off the bus a couple of stops early. Or even a couple of miles early.

Thankfully, Jeukendrup reckons office workers only need to up their daily energy expenditure by 500 calories: "That's probably an hour's worth of walking. But you could do half-an-hour of walking, and then be more physically active, instead of sitting down and watching television."

Doesn't sound too gruelling, does it? It's like the official Department of Health advice: adults should do 30 minutes of moderate activity (ie it makes you breath harder and, perhaps, sweat a bit, such as a brisk walk) at least five times a week. Perfectly sensible, and perfectly attainable.

Ken Fox, professor of exercise and health sciences at Bristol University, says: "We are not in trouble because people aren't going to the gym and playing sport - the reason we're in trouble, is we've stopped being active in our general routines."

Almost any movement is helpful - and for those of us with rather more than our fair share of the nation's collective tummy, walking is one of the best things we can do as it uses our own body weight.

But while that means a stroll round the shops can be good for you - although a brisk trot would be far more beneficial - you have not necessarily "earned" that piece of cake and glass of wine at the end of an exhausting afternoon. "It is dangerous if the reason you are increasing your activity is for energy balance," says Fox. "You can overestimate how much activity you've done, and underestimate how much you've eaten. The equation is always unbalanced - it gets biased towards what people want to hear."

Damn those exercise boffins, always trampling on our fitness plans. "We should be building exercise into our lives, and accepting it as part of our daily routine," Fox says.

You don't need to do it all at once - 10-minute bursts of activity are effective too. Half-an-hour of digging or shovelling in the garden can burn 250 calories, with 105 for weeding and 100 for raking. Moderate Hoovering, or mopping, will burn 160 calories an hour. Even clothes shopping is good exercise, especially when trying on different outfits - you can expect to burn around 100 calories an hour; 20 to 25 minutes washing and waxing the car can also dispose of 100. You'll also improve flexibility and upper body strength. And, of course, there's sex - half-an-hour of which can use up 120 to 180 calories. Kerry McCloskey, who wrote a book on the subject, claims she sometimes even clears the 350 calorie mark. "This is roughly the equivalent of briskly walking, running, or lifting weights for 30 minutes," she says.

But you do need to keep these things in check. Two hours of dancing on a Saturday night after downing your own body weight in booze, will not give you the figure of a dancer. Lucy Banfield is a member of the ensemble for the West End show, Dirty Dancing - and if you want be as fit as she is, you'll have to work hard. "When we're rehearsing we usually start at 10 and finish at six. It will be learning a new dance and going over it again, and again, so rehearsing can be harder than doing a show," she says. Even when she's performing in the evening, Lucy will attend dance classes and keep up with her Pilates.

So while it is perfectly possible to stay healthy and fit, without ever having to enter a gym - so long as you actually do walk to work, rather than just thinking about it - we shouldn't start closing down leisure centres quite yet. For a start, if you're looking to improve muscle tone, or add bulk, using specific gym equipment will be the most effective way. Second, the reason why we joined in the first place - to force ourselves to exercise more - is still an important factor. "At the gym there are more performance markers, so you can see your improvement because you can do something for longer, or do it better," says Jeukendrup. "It's a way of motivating yourself."

Signs of improvement are not so obvious in everyday life. Many sports scientists are reluctant to give average calorie-burning statistics on, say, vacuuming, as they differ wildly according to each person.

Be reassured, however, that panting up and down the office stairs - 15 to 20 minutes = around 100 calories - bottle of water in each hand, is in no way a waste of time.

See you in the top-floor meeting room for some quick star jumps.