'Mummy, your face is purple." It is a sad day in our house. I have been attempting to exercise, having utterly, completely failed to do anything about the extra stone that has stuck to my stomach following the birth of my son six months ago. My two-year-old is not impressed and wants to go to the park. The baby, having watched me heave my bulk round the front room trying to mimic Cindy Crawford, is now hysterical - and not with amusement. The only muscle likely to get toned here is my jaw as I clench it trying not to scream at my children. Post-partum "get back in shape" plans go out of the window. Before I know it we are in the park and a choc ice has appeared in my hand.

Which is nice. But what am I supposed to do about all the extra weight? Not care would be a good start. That worked for me for quite a while. But after months in drawstring trousers and huge knickers it would be nice to fit into some of my old clothes. The truth is that however much you claim to love your swollen thighs, the novelty begins to wear thin after a while. Even the sublime Ms Winslet, 25, now admits that she felt like "the back end of a bus" after the birth of her daughter Mia 10 months ago. "My bottom looked like purple sprouting broccoli," she confides to In Style magazine. "I was an absolute sight, I really was."

Well, there are sights and sights, but hey, at least I'm only looking at losing a stone - rather than the four Kate says she's lost with a mixture of a "personal eating plan" and two trips to the gym each week. One thing is certain however: it's not going to be easy. As Ms Winslet puts it: "People tell you your weight drops off afterwards. Hell-o? It does not."

"Breastfeeding burns around 500 calories a day," says dietician Sue Wood. Indeed, that's what pregnancy flab is for. You gorge yourself silly for nine months on Mars bars in the name of "cravings", then the baby is born and you convert it all to the extra energy needed to produce milk.

This worked for me up to a point. About a stone melted happily away. But the rest is going nowhere. Dieting, even if I was prepared to do it, is a bad idea when breastfeeding. "You absolutely should not go on a crash diet - at any point, but especially not after childbirth," says Wood. "If you aren't eating enough, your milk supply could go down." And you could deplete your body's store of nutrients (vitamin stocks and minerals, such as calcium, that have already been dented by pregnancy). You will also drain your energy (if you have any left after all the broken nights and strung-out days).

In fact, even if you ate a good, nutritious diet before and during the pregnancy, you will need a few months to restock all the nutrients your baby needed in the womb. And if you didn't eat particularly healthily before and during pregnancy (even if this was only because you were throwing up), it can take much longer - because, as Wood puts it, "the baby acts almost like a parasite: it will get all it needs at the expense of the mother's stores".

So what am I supposed to do? "You need to get a balance between healthy eating and activity," says Wood. In other words, cut out the choc ices and get some exercise.

I did try going to the gym. When I left the children in the creche, they howled so loudly that I had to be winched off my stepper to take them away. Of course, I could swim or jog, or do aerobics when my husband gets home but, frankly, by the time he gets through the front door all I want to do is lie down sobbing and drink seven gin and tonics.

I read in a magazine that skinny star moms such as Madonna and Uma Thurman did power yoga to get back in shape. So I go to the bookshop. The yoga moves all look perilous and extraordinarily uncomfortable. Since I do not have the luxury of a Hollywood yoga guru I look for something less challenging.

Walking, it seems, is not only do-able but "fat-burning". No equipment, and you can do it with a buggy. There are even exercise classes in the US called "strollercise".

The walking book is a total waste of money because the message could be written on a postcard: walk at a brisk pace for at least 30 minutes four or five times a week. Preferably do some sit-ups, push-ups and squats afterwards. This will strengthen major muscle groups and help you lose weight. (You will burn calories and also create more muscle, which in turn burns more calories than fat.)

There are various ways to tell if you are going fast enough. You have to get your heart rate up to 60-80% of its maximum capacity: to calculate this you take 200 and subtract your age. This is your maximum. Then subtract another 40. This is your minimum. Then you take your own pulse. If this sounds too technical, go for your "rate of perceived exertion", as Gillian Fletcher, obstetric physiotherapist and president of the National Childbirth Trust, puts it. Are you feeling warmer? Slightly out of breath? Good.

I pile the baby into his pushchair and stand his sister on the toddler step. All is hopeful for at least five minutes. I am lightly puffed - able to talk, but not sing as the book advises. Then my daughter wants to walk too.

She goes rigid and screams when I say no. We return to the house, slowly and in a very bad mood.

Catherine Zeta-Jones reputedly got on the treadmill the day after delivering her son. So maybe if I had started earlier it wouldn't be so hard. "You shouldn't start exercise classes until your six-week check, though walking is fine whenever you want," says Fletcher. What about sit-ups? "Not immediately, because your deep transverse muscles part during pregnancy," she says. Your hospital should provide advice on safe exercises to help them close.

Fletcher suggests exercise videos. I go to the library. Many have titles like Abs of Steel, From Baby to Bikini and Return to Slender. I grab one that I can do with the baby while the toddler is napping - Mommy and Me. I take Cindy Crawford, too, out of sheer self-destructiveness.

Things start well. My daughter is napping and my son is thrilled to be exercising with me. However, unlike the babies on the video who are soporific and often seen in soft focus, he screeches with raucous laughter each time I lift him or squat. By the time we get to our "abs", he is horribly overexcited and starts to bellow when I balance him on my stomach to do "crunches". I have to turn the tape off to try and calm him down.

Later, sedentary at the computer, I discover that it's not all bad news. Babycenter.co.uk claims that you should expect it to take 10 months to a year to return to pre-pregnancy weight. And my husband, who was also, inexplicably, eating for two during the pregnancy, has now agreed to go for walks with me after work. With two "strollers" there's a fighting chance that we'll manage some strollercise. It may not be astanga yoga or Crawford's personal trainer, but it's got to be a better way to lose weight than drinking gin and tonic.