What are the health benefits of running?

The benefits of vigorous exercise are well described. The American College of Sports Medicine Position Statement on Exercise is a document chock-full of studies proving that vigorous exercise yields plenty of health benefits. One of the major points of the position statement is that there is a dose response to exercise; that is, the more you do, or the harder you do it, the more benefit you accrue. But this point is not to discount moderate exercise. You get plenty of benefit from moderate exercise, it's just that vigorous exercise seems to accrue even more benefit. The ACSM report makes it clear that "many significant health benefits are achieved by going from a sedentary state to a minimal level of physical activity; [but] programs involving higher intensities and/or greater frequency/durations provide additional benefits. For example, it was shown in one study that individuals who ran more than 50 miles per week had significantly greater increases in HDL cholesterol (the good fat) and significantly greater decreases in body fat, triglyceride levels, and the risk of coronary heart disease than individuals who ran less than 10 miles per week. In addition, the long-distance runners had a nearly 50% reduction in high blood pressure and more than a 50% reduction in the use of medications to lower blood pressure and plasma cholesterol levels."