Mostly flat loop course, apparently "precise to the millimeter." I dug up this Letsrun thread (
http://www.letsrun.com/forum/flat_read.php?thread=2766272 ) about the race, and this very same topic actually came up on the Fast Run Blog board a few years ago.
Imagine if you took NCAA XC Nationals, added in all the pros who race on the roads, and made it a half-marathon road race. You'd probably get something similar. But the depth in Japan is still definitely staggering.