Keep in mind I'm not in the liberal arts, nor do I have any real experience in writing or reading papers in such a subject. That being said, my first thoughts having read the paper (that haven't already been mentionned by previous posters) are these...
I'm not entirely sure you convinced me, as your audience, that there really are these definitive peaks and valleys in American distance running - you simply stated how the team did as a whole from Olympiad to Olympiad. I got the impression reading the paper that it was much like "in Olympics X, Americans placed A, B, C...in Olympics Y, Americans played D, E, F, without hypothesizing (convincingly anyways) WHY there was a peak/valley in performance that particular year. Is it coincidence? Environment? Politics? Coaching? Training? etc, etc, etc. Being a fan of the sport, I would have expected a history major, receiving funding for this type of project, to tell me more about the history of the sport than what I already knew from reading a few running novels and watching the sport on TV...and it doesn't appear I'm much older than you.
I'm more convinced that the sport in the U.S. is not much different than any other developed country of the world - each experience their own peaks and valleys for reasons probably no one can predict or describe, most of which probably aren't significant or due to pure chance alone. What about the growth of the sport around the world as a whole?
Clearly more countries take the sport seriously now than they did in the late 50s and early 60s. Could this be why we have seen a decline in performance recently?
All in all, a very well-written paper. I just wished you had hypothesized a bit more regarding any trends in performance and found concrete evidence to back it up. I'm afraid, however, I wasn't entirely convinced.