"The best evidence from science and the real world suggest that the more we run, the better we race". Full article here >>> http://running.competitor.com/2011/08/training/are-you-running-enough_35813. I link to this website on the side-bar, but the site is updated so often, good and interesting articles like this one appear listed there for a very short time only. Another quote from the short article: "runners keep improving as they increase their running mileage all the way until they get to one step short of the point of breakdown". But how do you know when you have reached that point? The article suggests that point moves out anyway as you get fitter, so by then you can run further. The article doesn't say what happens as you get a lot older, but you and I know that at some stage the point moves back in again. In terms of speed, I am capable of a lot less now than when I started running nearly 50 years ago. I wonder what the limits are in terms of distance? My tentative answer is, I can still run for as much time as I used to, but the speed and therefore the distance is necessarily less.