Odarumi MTB circuit (235km, 4655m climb, 14hr)　「大弛峠」

To complete this full circuit is more of an expedition than a cycle. You will need to either start or end in darkness - probably both.
A more sane course would be to get an early train to Enzan and start from there, still leaving an extremely long hard ride to Chichibu.

I ended up cycling all the way down from Mikuni-toge in darkness the first time I completed the whole course - there was one section where it appeared that the road suddenly ended as a landslide had hidden the way and it took some time to negotiate my way in the darkness back onto the road beyond the landslide again.
On more recent adventures, I opted to cycle to Takao (1hr cycle from Ome) and take an early train from there to Enzan instead.
Another alternative would be to cycle on Koshu kaido instead of Ome kaido to reduce the initial climbing.
On the way back, if you still have plenty of energy, you can choose the more remote route from Urayama to Naguri via Arima-toge (see [link TBA] for a circuit including this section) and avoids R299 which can be rather busy with traffic. This is shorter but you need to climb 900m instead of about 400m.

Category: Road: 203.75km, Dirt: 31.25km

Route: Ome, Okutama, Taba, Yanagisawa-toge, Enzan, Fruits Line, R210, Odarumi-toge, Mikuni-toge, Nakatsugawa, Chichibu, Yamabushi-toge, Naguri, Ome

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Notes:
The initial descent from Odarumi is snow covered from Dec to May and sometimes Nov and Jun as well. Some deep areas are tough going, but for the most part you can cycle/slide down in late spring..
Not often that you get to cycle 2 different routes in different prefectures but with same route number (R210 in Yamanashi and Saitama)!