Dr James O'Keefe from St Luke's Hospital (USA) Kansas has published some research that
Physical exercise, though not a drug, possesses many traits of a powerful pharmacologic agent. A routine of daily physical activity can be highly effective for prevention and treatment of many diseases, including coronary heart disease, hypertension, heart failure, and obesity," says lead author James H. O'Keefe, MD, of Saint Luke's Hospital of Kansas City, MO. "However, as with any pharmacologic agent, a safe upper dose limit potentially exists, beyond which the adverse effects of physical exercise, such as musculoskeletal trauma and cardiovascular stress, may outweigh its benefits
http://www.sciencedaily.com/releases/20 ... 093108.htm
In other interviews Dr O'Keefe suggests
Endurance sports such as ultramarathon running or professional cycling have been associated with a five-fold increase in the prevalence of atrial fibrillation, or abnormal heart rhythms.
Extreme exercise is not really conducive to great cardiovascular health. Beyond 30-60 minutes per day, you reach a point of diminishing return
I'd average a couple of hours every second day, but this has got me wondering - would it be smarter to back-off to something under an hour a day ?