Kunsunoke has excellent advice. Get insured. (And remember: Though people riding step-throughs are supposed to be on the left side of the pavement, they could be on the right, they could be on the sidewalk, the could cross the road without looking to see if it's safe.....)
You do not need a folding tourist bike in order to ride the train. Just remove the front wheel and attach it to the right of the frame, turn the handlebar to the right, and toss the laziest kind of bike bag (in Japanese, rinkōbukuro) over the result, and carry that (at the extreme front or extreme rear of the train). Your local station is the very convenient Haijima station.
But actually you don't need to take the train. Fussa (which is where Yokota is) is wonderfully situated. Skip Haijima station and instead just ride your bike a short way west to Itsukaichi, then take your pick of directions. Most people go west to Hinohara and then up to Tomin-no-Mori and Kazahari pass (the highest point on any road in Tokyo), but there are plenty of alternatives. Low gears (that's me) or strong legs highly recommended.