I know that it obviously makes you faster (well, generally speaking anyway), but what exactly is the mechanism involved that makes you stronger from higher mileage? A stronger heart? Stronger legs? Better running technique? Higher v02 max? All of the above?
Just wondering. I did my first well-paced 90 minute run the other day for the first time in a while (I had been focusing on shorter speed stuff), and already I can notice a huge boost in my fitness. I'm currently only running about 60 mpw, but I think I'll try to get it up to 85-90 by the time April rolls around.