Current government physical fitness guidelines state that: 1) the majority of the health benefits from physical activity can be obtained by walking two miles briskly on most days of the week; and 2) the health benefits of physical activity depend principally on the total amount of activity rather than the intensity of the activity. Nevertheless, there are currently no prospective epidemiological studies extant, designed specifically to directly contrast the health benefits and costs of moderate exercise (e.g., walking) versus vigorous exercise (e.g., running).