Passed on from Aryk is How You Live Is Your Choice To Make which has this and other related gems:

Q: Doctor, I've heard that cardiovascular exercise can prolong life. Is this true?
A: Your heart only good for so many beats, and that it...don't waste on exercise. Everything wear out eventually. Speeding up heart not make you live longer; it like saying you extend life of car by driving faster. Want to live longer? Take nap.