Pretty much everyone I know who has done a lot of running has needed replacement knees and hips in later life. Those endorphins ( runners high) that circulate in your body are just to mask the damage being done to joints in lower body due to impact. I saw a report many years ago which investigated the whole body fitness of various athletes, and runners and cyclists were way down the list due to the low number of muscles they use, cross country skiers were the fittest followed by swimmers, because they use pretty much every muscle in their body, but at low impact to their joints.