I've ridden bikes all over japan, not touring style, but just ridden. You are not likely to get stopped by Japanese police while on your bike unless you're being stupid. But they do have 'meter maids' that go around and check for the registration stickers when bikes are parked. Just ask your friend where to get it registered. I bought a cheap bike from a store and it came registered. And make sure you park in designated areas, or you will get a ticket. It's a sticker and a pain to get off. I never paid them, but they didn't know who I was anyway.
I don't know much about the Kanto region either, but I did a mini tour (3 days) across Hiroshima on a used kids bike that I bought for $15 at a recycle (pawn/second hand) shop. I was originally going to set out on the main highway, which I think it illegal and some drunk old guy advised me to take the secondary highway instead. Much safer, and better scenery. Glad I took his advise. Kinda the equivalent of Route 66. Take the non-tolled highways in Japan and you should be able to travel efficiently from point a to b, with safety and great scenery. If you'll be travelling over the course of a few days, bring a tarp or tent. Depending on the season you are going, I would advise you to be aware of rainy season in Japan. A month or so of almost non stop rain.
and if you're bike is too big for the car, consider asking your friend to skip the car and take the train with you and help you carry your luggage. There is the Narita express (slightly more expensive than the regular train), which hits all the major way points to transfer and get you anywhere you need to go in the Tokyo region and beyond.
Last edited by Maldivejoe; 05-25-14 at 02:16 AM.