It's easy to ignore an exercise regime when you think you're already fit, says Sam Murphy, but it's great insurance for the future
You're in your 20s - you never seem to stop running around, you eat what you like without gaining weight, your energy levels are high and your health seems great - so why bother with exercise? Bob Smith, director of analysis and performance in sport at Loughborough University says 'this is the decade when you can develop maximum strength, endurance and flexibility, which you'll find much harder to achieve later on.'

The time is now
Starting good habits now bodes well for future health. Many of us forget that the purpose of exercise is more to do with health issues rather than fitting into size 10 jeans. You might not have to worry about your weight now, but statistics show that as a nation we're getting fatter. More than half the adult population is now overweight with more than 10 per cent of men and women classified as obese.

And with that comes a host of health problems, including a higher risk of coronary heart disease, high blood pressure, high cholesterol levels and diabetes. Severely overweight people are also more prone to musculoskeletal problems, such as osteoarthritis and back pain.

'It's not so much what we're doing that's causing the problem, as what we're not doing,' says Ben Wilde, a sport psychologist and group fitness manager for Esporta Health Clubs. 'People are becoming increasingly inactive in the UK.' Research shows that we are eating an average of 500 calories a day less than we did in the 1950s, yet people are getting heavier. The reason is that we use our bodies less, and expend less energy on a daily basis.

Learning to love exercise
But what if you hate exercise? 'If you don't enjoy working out, it must be because you're doing the wrong thing,' says Wilde. 'The most crucial aspect of any exercise programme is fun, so that you'll stick with it.'

Aside from personal preferences, what type of exercise is best? 'The more cardio, the better,' believes Bob Smith. In other words, cardiovascular exercise, the type that gets your heart and lungs fit and burns calories fast. It could be anything from dancing or cycling to soccer or orienteering.

In fact, varying activities is beneficial, according to a report in Peak Performance magazine, which stresses the importance of cross-training to avoid overuse injuries, boredom and burnout. 'Developing good cardiovascular fitness in your 20s gives you a solid base on which to build later,' adds Smith.

Look after your bones
Strength training, working with either free weights or your body's own weight, has a number of potential benefits. Not least, it helps control body fat and enhances muscle tone. Even just one set of 12-15 repetitions of an exercise for each of the body's major muscle groups can have an impact on your body shape.

Another plus of regular strength training is the beneficial effect it has on bone. Until the third decade, bones - while fully grown in size - are still getting denser. The late teens and early twenties are a 'window of opportunity' to lay down bone, getting them as strong as possible in order to prevent the bone-thinning disease osteoporosis and related frailty and fractures in later life. Intense bursts of high-impact aerobic exercise can also stimulate bone growth in the lower body. One recent study by Dr Joan Bassey found that 50 jumps on the spot a day increased bone density in women.

Don't overdo it
So do you need to become a fitness fanatic to reap the benefits of exercise? Not at all. Three to five sessions of cardio work a week at a moderate intensity is sufficient to strengthen the cardiorespiratory system and improve body composition. Add in two or three days of weight training with a day off between each session and you'll see results fast.

But don't think that just because some is good, more must be better. Even if you're trying to lose weight, exercising seven days a week or for hours at a time can overload the body and has a detrimental effect on your health. It can hamper the immune system, leaving you susceptible to colds and flu and other infections. If your periods stop for more than three months outside of pregnancy it may be that you are overexercising, and have developed amenorrhoea (absence or stoppage of your period). This can have serious implications for your bone health later in life, and it's essential that you see your doctor.

Where do I start?
If exercise isn't already a part of your life, now is the ideal time to make it so. Don't get bogged down with boring, repetitive routines - have fun and try new things. 'We have an expression about exercise: "take it regularly, not seriously"' says Andrew Richardson, director of the Australian Fitness Industry Association. 'For exercise to have a real influence on your health, it's got to be a lifelong commitment.' So, whether it's kickboxing, spinning, salsa or circuit training - if you want a great body, optimum health and a long life, start now.