Cool.
I lived in Japan from 2001 to 2011 and rode XC/DH and everything in between for all of it. I did some S24O rides in Aichi and Shizuoka as well as a few small tours ( around Biwako, one overnighter to Shikotsuko in Hokkaido when I was living and working in Niseko in 2008, 3 days in Nagano - steep!) and always meant to do more. Sadly life got in the way before I did a big ride and now I'm back in Australia.
Looking forward to reading about more of your adventures. If you can get to the Pacific coast, I highly recommend the Tokai Nature Trail (東海自然歩道) - it is fantastic riding and stretches barely interrupted from Tokyo to Osaka.
Cheers,
Chris