Research published recently in the Annals of Behavioral Medicine showed that the harder people exercise, the less pleasure they feel during the exercise and the less likely they'll be to exercise routinely. According to one of the authors of the study, "Evidence shows that feeling worse during exercise translates to doing less exercise in the future."
Obvious, right? We tend to do things we find pleasurable and avoid things we find painful. If you want to stay committed to a long term exercise program, don't make it so hard.