More on the Value of Exercise
Level of exercise beginning in middle age and mortality rate are very well correlated, as shown by this study: "Despite the known hazards of physical inactivity, it continues to be a major health problem. Physical inactivity is associated with increased incidence rates of obesity, diabetes, cardiovascular diseases, osteoporosis, and cancer ... The absolute mortality rate was 27.1, 23.6, and 18.4 per 1000 person years in the groups with low, medium, and high physical activity, respectively. The relative rate reduction attributable to high physical activity was 32% for low and 22% for medium physical activity. ... Increased physical activity in middle age is eventually followed by a reduction in mortality to the same level as seen among men with constantly high physical activity. This reduction is comparable with that associated with smoking cessation." To restate that last point: lazing around and skipping over a little exercise every day costs you as much healthy life in the long run as smoking does.
Link: http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=2654773