This thread has a lot of good comments and some near misses. Japanese love the sport of distance running, but beyond high school, the sport moves in a different vector, and in my opinion, that accounts for the difficulty in comparing the USA and Japan. Contributing factors: Japan's under-30 population is merely 1/4 of the USA (not 1/2). Foreign athletes and athletes older than 18 (high school) and 22 (college) are very rare. The love of ekiden pulls athletes away from what Westerners consider to be traditional middle distance competition.
Japanese do funnel athletes (by free choice) to high schools with highly effective coaches. And Japanese HS track athletes do achieve great marks in all events, but especially middle distances. But after that, the sports-in-college system is very different from the USA. Only a few colleges have full-scale track teams; more have ekiden teams, and the best ones are in a few universities in the Tokyo area. In either case, few have track or ekiden programs for women. The inverted funnel narrows dramatically.
So, unless you are male and already very accomplished at long distances by age 18, the number of opportunities to train and compete in a wide range of track events nearly disappears after high school. The best option for girls is to get on with a professional ekiden team. Having moved up from high school, professional women runners and male college (and corporate) runners shift their focus to ekidens, half marathons and non-traditional distances, in pursuit of regional and national championships.
These are factors why Japanese collegiate and professional athletes, then, are conspicuously absent from world statistics and Olympics, etc.. High-school burnout and excessive over-distance training in college are also factors. At least, that is my opinion, from teaching and coaching/officiating in Japan for many years.