The pressures of home and family life can also mean it feels as if there's little time left to fit in exercise. It's certainly tough to get started.
So, it's worth thinking about what you gain from regular exercise and making even a partial improvement to your fitness.
Physical inactivity is an independent risk factor for coronary heart disease - in other words, if you don't exercise you dramatically increase your risk of dying from a heart attack.
Conversely, exercise means a healthier heart because it reduces several cardiovascular risks, including high blood pressure Being physically active can bolster good mental health and help you to manage stress, anxiety and even depression.
Regular exercise can help you achieve and maintain an ideal weight, which can be important in managing many health conditions, or may just make you feel happier about your appearance.
All exercise helps strengthen bones and muscles to some degree, but weight-bearing exercise, such as running, is especially good in promoting bone density and protecting against osteoporosis, which affects men as well as women.
Different exercises help with all sorts of health niggles, such as digestion, poor posture and sleeplessness, and physical activity can be beneficial for a range of medical conditions, from diabetes to lower back pain.
There are lots of positive reasons for getting fitter, including meeting new people, discovering new interests and generally feeling better, but if you need to be scared into doing more exercise, consider the following:
On current trends a third of men will be obese by 2010, according to a 2006 Department of Health report.
Between 2003 and 2006, obesity in adults rose by nearly 40 per cent The picture is just as worrying for youngsters - by 2010, it's predicted 22 per cent of girls and 19 per cent of boys between the ages of two and 15 will be obese, with girls under 11 at particular risk.
Obesity is responsible for 9,000 premature deaths a year in this country, and is a major contributory factor to heart disease Coronary heart disease (CHD) is still the leading cause of death in the UK, accounting for about a fifth of all deaths, according to the Office for National Statistics.
About a third of deaths caused by CHD are among people aged under 75
Almost half of adults in the UK will be aged over 50 by 2020. We tend to assume the benefits and pleasures of sport, exercise and fitness are only for younger people, but think again. The rewards of improved fitness later in life can be great both for your health and social life.
Statistics show activity levels decline steadily with age, and by their mid-50s few people take regular exercise.
But regular activity is especially important as you age because it has beneficial effects on conditions such as diabetes and cardiovascular disease, and helps you maintain mobility and mental well-being and, consequently, your independence.
There's no reason you should give up the sport you love just because you're getting older. There are plenty of exceptions to the statistical trend of decreased activity as we get older at clubs up and down the country, for example, there are runners in their 50s, 60s and beyond whose fitness puts people 20 or 30 years their junior to shame.
And even if you weren't especially active or sporty at a younger age, it's never too late to start. Male or female, single or with a partner, there's lots you can do, and enjoy.
Some of the health benefits you'll get are the same as younger people, but there are things that are of particular benefit as you get older:
More energy - exercise makes you feel more energetic, while sitting around not doing much makes you feel sluggish and unable to do anything.
Improved sleep - your body and mind feel as though they've done something and are ready for rest at night.
Stable weight - regular exercise helps to keep you at a healthy weight.
Improved circulation and lower blood pressure.
Delayed ageing - keeping active strengthens your muscles, joints and bones as well as helping with mobility and balance, important as it helps to prevent falls, which are the leading cause of injury and death for the over-75s.
On top of the health benefits, exercise can be an excellent way to meet new people, whether it's at a gym, a rambling or running club, or just people you meet while walking the dog.