The debate between the relative benefits of moderate vs. vigorous exercise is ongoing. Federal guidelines encourage a minimum of 30 minutes of moderate-intensity exercise most days of the week, while earlier guidelines suggested vigorous exercise for at least 20 minutes three or more times per week.
A study in The New England Journal of Medicine compared the incidence of heart attacks and strokes in approximately 75,000 women ages 50-79, based on total time spent doing any physical activity; walking; vigorously exercising; and sitting. Activity was determined by questionnaires, and classified as mild, moderate or vigorous.
The more physical activity completed by the women, the less likely they were to suffer from heart attack or stroke over the next six years, regardless of race, weight or age. Women who walked or performed vigorous exercise 2.5 hours or more per week reduced their total risk for cardiovascular conditions by approximately 30%. Brisker walking pace and less time spent sitting daily reduced the risk even more.
It all adds up: The more time you spend being active - a little here, a little there - the healthier you will be. Ideally, you should try to complete aerobic workouts at a moderate to vigorous pace, but don't give up altogether if you can't accomplish this. Simply be as active as you can, as often as you can.
Manson JE, Greenland P, et al. Walking compared with vigorous exercise for the prevention of cardiovascular events in women. The New England Journal of Medicine 2002:347(10), pp. 716-725.
Other studies on sports and fitness can be found at http://www.chiroweb.com/find/archives/sports.