I meet Denise Lewis in the cafe of her local gym, with the whomp-thump-whomp of dance music pulsing overhead, and an army of frenzied limbs of a class thudding rhythmically just a glass wall away. It's an appropriately high-energy place to interview the one-time heptathlete, who bounds in with her son, Ryan. We are here to discuss her new book - a seasonally apt guide to flattening one's stomach - flicking through which, we quickly arrive at a close-up of Lewis's torso. Light licks her stomach muscles, which bulge and recede in all the right places: a ripple here, a furrow there. I am suddenly more aware than ever of my stomach lolling flumpishly atop my thighs. "Wow," I whistle quietly.

Lewis tuts unhappily. "This is what my stomach's like now. But before, when I took a picture, you'd see everything - every detail, every muscle." I peer closer. There sure seem to be a lot of muscles there to me. "Oh, well," she laughs, "I guess it's not bad. I'll take it!"

If Lewis is fastidious about her physique, it's hardly surprising. In the heptathlon - hurdles, high jump, shot, 200m, long jump, javelin, 800m - Lewis's muscles powered her through, while also serving as a warning to her opponents. On the warm-up track, she says, "People would measure you by your shape," which is why "you used to find a lot of male athletes peeling off their tops - just to say, 'Have some of this.' I guess my abs were a sign to people. 'Watch out, she's in shape.' "

Lewis had her first child, Lauryn, while still competing in 2002, and during that pregnancy "I didn't exercise like crazy, but I was in the gym. I was still in trim and meticulous. I had to be." After retiring in 2005, though, she loosened up. She had ditched junk food more than a decade before, but now "I was almost like a teenager gone mad. I thought, 'Wow, I can do this again! I can eat as much as I want to, and I can drink!' It was complete indulgence." In 2006, she had Ryan, and after his birth, "When I needed my fix, I'd go for chocolate digestives - those big, supersize packs."

She had grown so used to having a flat tummy, it came as a shock when, one day, she found it wasn't there any more. "During my career I had been training twice a day, every day, because it was all about me. That was the job. But with kids, my hours were vastly reduced. Normal life took over. I started to understand what people meant when they said, 'I can't find the time.' At the end of every day I was exhausted."

All of which meant that Lewis saw in the last new year feeling unhappy: "I just didn't feel in control." Her answer was to set herself specific, achievable goals. She began exercising regularly, although she claims her current shape is the result of just three sessions a week. "Once I'd got that back on track, I had the endorphin kick I needed to do something about my eating. I felt much more positive."

Her book contains modified versions of the exercises that have toned her own torso, and while it might seem a little gentle and obvious to a regular gym-goer, it's good, straightforward advice for beginners and those returning to exercise. It also helps that Lewis is a more informed guide than some of the other would-be exercise gurus, and I can vouch for the fact that the combinations of knee lifts, trunk rotations and back raises penetrate deep into the abdominals. ("That's how you know it's working!" she says cheerily.)

Lewis has always been good at sticking to her goals. Born to a 17-year-old single mother, she grew up in Wolverhampton, where her teachers initially suspected her force of will would make her the country's first female prime minister. While Margaret Thatcher stole that trophy, Lewis began to prove herself as an athlete. Come adolescence, she was attending her local comprehensive and travelling to Birmingham - a three-hour round trip - four days a week to train. By 14 she was English schools long jump champion; at 17, her coach suggested she try the heptathlon. At the Atlanta Olympics in 1996, she was the only British woman to win an athletics medal, taking home the bronze.

As far back as the early 1990s she had targeted the 2000 Sydney Olympics as "her baby". Eight weeks before the games, though, she was badly hurt, and struggled through the competition with foot, ankle and calf injuries. It took everything she had in her last, least favourite event - the 800m - to take gold, and she puts the victory down to mind more than muscle. "When I think about it," she says now, "I get tingles, because it was only my mind that got me through that."

What are Lewis's tips for those of us who are less naturally resolute? She suggests writing down aims and progress ("On paper, not the computer - you'll just get distracted by emails"); concentrating on one or two goals at a time; and establishing a definite routine.

Does she think there is too much pressure on women to regain their shape after pregnancy? "The magazines drive me insane," she says, "because it's such an unrealistic ideology, that you're going to have a baby, or even just be out of shape for a little while, and then ping back into shape. I would encourage any woman not to buy into the hype. You don't really know what celebrities are doing to get their so-called body beautiful back so quickly, and you have to be realistic - there are inevitable changes in your body when you have a baby. It takes nine months for you to put the weight on, and you should give yourself at least that time to lose it again."

Along with looking after her kids, she is embarking on a Strictly Come Dancing tour later this month, working on a project with the British Olympics Association - "It is all about the Olympics for me. Almost my being" - and improving her latest enthusiasm, golf: "At the moment I'm still in the learning phase, so I'll be writing a little journal about what I've learned and what I need to implement for the next time." Does she have any specific new year's resolutions? With a diamond-glint of a smile, she intends, she says "to be even more ... not proactive, that's not the right word. More ... in control. Sounds dangerous, doesn't it?"

· The Flat Tummy Book, by Denise Lewis, is published on Thursday by Quercus Publishing, price £12.99. To order a copy for £11.99 with free UK p&p, go to theguardian.com/bookshop or call 0870 836 0875.