Have a great trip. I used to live in Sapporo, and travelled much of Hokkaido by bicycle. And also cycled through much of the rest of the country, too. Each island several times.
Japan is a great place to travel / cycle. There's always some place to pitch a tent, take a dip in an onsen (or sento) and find some good food and beer. Even if it's only from a vending machine.
Onwards!!