The College of Sports Medicine in San Francisco recently reported that in a large study of adults over a 30 year span, those that reported that they ran for exercise had about a 19% lower death rate than non-runners.
But, more interesting was that those that ran just 1 to 20 miles per week, at an average pace of 10 to 11 minutes a mile decreased their risk of dying more effectively than those that were sedentary OR those that ran more or at a faster pace. Such moderate but steady exercise, like jogging or walking, is said, on average, to increase the life span for man by 6.2 years and a woman by 5.6 years.
Other studies have found that more aggressive exercise programs may provide additional benefits but plateau rapidly. It seems that at a higher level we work harder for less overall beneficial gains. Slow moderate exercise is the "sweet spot" for getting the most out of your effort.
That's good news for a lot of people and it confirms to me the benefit of even moderate or occasional exercise. - but I still like to push harder myself ;-)