This ride is awesome, especially for families. If you research where the Uraminotaki (Urami Waterfall) is and obtain directions, first the waterfall is awesome to see and then coming down the mountain is even better with all of the evergreens. The trail is very easy to find right off the road next to the camp grounds and can be followed all of the way down back to the lake road.
There are in-fact a ton of trails in this area and I had some difficulty finding some of them. MTB Japan provides tour guides around this area, the guys are fluent in both Japanese and English, and carry all of the equipment needed to include bikes, helmets and everything else. Check out their websites: