The Long and Short of It
An estimated 60% of U.S. adults fail to participate in regular physical activity, and nearly one in four aren’t active at all -- 
despite the established associations between exercise and the risk of coronary heart disease (CHD), cancer, and numerous other life-threatening conditions.
If time and/or energy constraints keep you from exercising, take note of the results of the Harvard Alumni Health Study. More than 7,000 healthy men reported the number of city blocks and flights of stairs they walked each day, and their weekly participation in recreational activities (frequency and duration).
Follow-up questionnaires were used to determine how many men had developed CHD or related heart conditions.
Exercise duration had little or no effect on CHD risk, after adjusting for total energy and other variables. Specifically, longer sessions of exercise were no more effective than shorter sessions in reducing risk, when total energy expenditure remained constant. So get off that couch and start exercising! If multiple short exercise sessions are as efficient as a single long session, you’ve got no excuse for staying inactive.
Ask your doctor to help outline a fitness regimen suitable to your needs.
Reference:
Lee I-M, Sesso HD, Paffenbarger Jr. RS. Physical activity and coronary heart disease risk in men: does the duration of exercise episodes predict risk? Circulation 2000: Vol. 102, pp981-96.
