I remember reading something in Scientific American (in the 70's?) about the progression of records. I can't remember the details, unfortunately, but the conclusion was along the line that we aren't running quicker, as such (allowing for improvements in shoes, tracks, diet, etc) - but, rather, that we are able to run at the same speed for longer. For example, the average speed per mile for the world record marathon is roughly 4min 35sec per mile. This is the sort of time per mile that the top runners were doing for 10 miles only 40 years ago.