Every race you run will use 100% of your available slow twitch muscle fibers (cells). These are your most "aerobic" cells, meaning that they can produce more aerobic energy than your faster muscle fibers (intermediate fast-twitch and fast-twitch). It therefore behooves you to develop them to their full potential, so that they can last longer in a race. When you train faster than distance pace, however, you also start to use your faster fibers at an increased rate. Since these fibers are less aerobic, they fatigue more quickly, limiting the duration of your effort. This, in turn, limits the volume of training that your slow-twitch fibers receive--thus limiting the development of capillaries, mitochondria, and increased glycogen (carbohydrate) storage within your slow-twitch fibers. On the other hand, if you dial back the pace and go long, you can give your slow-twitch fibers the kind of workout they need to develop to their maximum potential (well, of course, the faster work provides some important stimulus and subsequent adaptation for these slower fibers, too, but the slower work is invaluable). Also, as already mentioned above, distance runs at a slower pace develop "running economy"--this is the practice makes perfect part of running. Your nervous system becomes incredibly efficient at triggering your stride ... step after step after step. You can't train hard (fast) more than about two to three times per week before it becomes self-defeating, so use your other days to build the "base" of incredible slow-twitch endurance that will serve you well in the future. Good luck!