"But the quickest way for U.S. athletes to get good is that they have to run a lot. They have to run aerobically at high intensity and have quite a lot of accumulated mileage. That is the only way we are going to catch up to the Africans, because they are so far ahead of us. So it’s got to be large volume, but not just that; it has to be long, hard stuff that raises threshold."-Brad Hudson, former coach of Dathan Ritzenhein responding to a question about whether Ritz's marathon training led to his recent breakthrough 5k PR.
I havent read Run Faster, Hudson's book, but I really like this quote because it emphasizes the same principles that Jack Daniels recommends in his bible, Daniels' Running Formula. Namely, lots of work around your "threshold" or maximum lactate steady state (MLSS) is the best way to improve fitness. While everyone loves to go to the track and sprint some 400s, the real benefits come from running at a slightly lower pace for a much longer time, either through tempo runs or intervals with short rest. Doing copious amounts of work at this intensity will allow you to drastically increase your fitness with a much lower risk for injury.