The hordes of Americans who roll out of bed,slip into their Reeboks and run foran hour in the face of snarling dogs,potential muggers and hordes of Americans heading in the opposite direction on thier Schiwinn 10-speeds must wonder sometimes whether it's worth the aggravation.After all,if a rash of recent books and articles like "The Exercise Myth" can be believed,the evidence that physical activity leads toa longer and healthier life is based on a flawed interpretation of cause and effect.It isn't the exercise prolongs life,the argument goes,it's just that people who engage in sports and active occupations are healthier in the first place.But the fitness buffs should not put their rowing machines in dry dock just yet.According to a long-term study involving nearly 17000 loyal sons of Harvard,it now seems that athletic effort is far from a waste of time.Moderate exercise,said a report in last week's New England Journal of medicine,can add up to two years to a person's life.