The point is that the exercise in the experiments I heard about was FORCED exercise. The rodents were given a mild elactric shock if they slowed down(poor things) and the humans were originally on a tandem and are now on a biike that sets the pace.This means that they are exercising well out of their comfort zone.In a marathon you are exercising well out of your comfort zone.In daily exercise, if the majority are like me and not my husband, you push yourself but never to the stage of being really uncomfortable Ihave seen my husband finish a marathon, when he has had a bad one, nearly on his knees.Now he says it is not worth exercising if you do not have a fair degree of discomfort.Do not ask me what his motivation is as I do not know. Yes,you can speculate!!!!!.