Longevity and Exercise are inter-related but you need not be a marathoner to achieve good, prolong health. In fact, excessive, intense exercise has been shown in most studies to be detrimental to health in the long run. It increases morbidity and mortality.
Listen to Dr O 'Keefe on this issue:
....my own personal addition to this is from the Islamic perspective.
Don't just exercise...do something to your mind while you are burning all those calories and moving all those muscles !