The doc said that you only have so many miles in your knees and you can use them up running or you can be walking around on your original ones come retirement age. The guy across the street is a runner and I'd see him around town all bent over struggling to stay upright as he plodded along such that I'd want to give him a lift back home. I told him once that running must be a lot like pounding your head against a wall because it feels so good when you quit.
