Cycling is an excellent form of aerobic exercise. People who exercise for 30 minutes a day have a greater life expectancy and lower risk of disease than those who are sedentary.  Regular exercise reduces the risk of coronary heart disease by half. Other diseases such as high blood pressure, osteoporosis, diabetes mellitus and depression are also less frequent among people who exercise regularly. 
If you have access to quiet roads or cycle paths in your community, it can be an easy way to exercise while also saving time; for example, by cycling to work rather than being stuck in a traffic queue.
Cycling to work, school or the shops is a great way to get fit, feel good and manage your weight. Modern bikes are lightweight and many are affordable. They can also be fitted with panniers and baskets that can carry a surprising amount; a lot easier to carry on a bike than on foot.
While the British weather can sometimes be intimidating to first-time cyclists, what looks like a shockingly drizzly and cold day from within a car can be refreshing on two wheels. You don’t have to get hot and sweaty, just ride at your own pace. Pushing your bike up a hill still counts as exercise, and if it’s got you out of a car then you are saving the environment too.
 Powell KE, Thompson PD, Caspersen CJ, Kendrick JS. Physical activity and the incidence of coronary heart disease. Ann Rev Public Health 1987;8:253-87. [Medline]
 Berlin JA, Colditz GA. A meta-analysis of physical activity in the prevention of coronary heart disease. Am J Epidemiol 1990;132:612-28. [Abstract/Free Full Text]