Zat0Pek, more than anyone else on this board, has been correct in saying the problem with American distance running (among many) is we encourage sucessful runners to compete at the shortest distance possible. Further, he says if Geb was an American, he would have been a very good miler and not the truly great of all time.
Ryan Hall, in this interview, supports this contention. He says that 2 years ago he would hve never envisioned himself as the AR holder at 20k and the half. Two years he said he was a frustrated miler, 5k runner.
http://www.flocasts.com/flotrack/index.php?n=117
Maybe with the sucess of Teg, Hall, Ritz moving up, even Webb we finally seeing this mentality change. Not ever sucessful distance needs to give it their all at the mile first and only, then move it when nothing else works.
Find what seems to be your nature distnace and compete at that level starting at 18. Don't believe the marathon/10k is what you do when your 28 at the end of your career.