Can you give details as of where those roads and single track is ?
I am less then 3 month in Yokohama (Bluff area)... and also in search of MTB \ downhill trails.
I've seen a guy in full face helmet and armor near the Homes\OK and since then wondering where he was riding
So far I found 2 places to downhill
FujiPanorama (http://www.fujimipanorama.com/summer/) and FujiTen (fujiten.net)
been @ first on , not impressed (I am spoiled by keystone, CO)
and it's cash only
BTW there is a 2010 demo there this weekend (http://yinternational.heteml.jp/ys-mtb/2010/09/post-20.html)
if google transalte toolbar did it work right