I’ve lived in Japan for coming up to 12 years, I ride and train 5-6 days a week regardless of weather conditions, have toured up to 300ks a day for 7-10 days at a time and from reading your ideas and expectations regarding what you’ll be carrying on the bike, available facilities and restaurants/convenience stores and your planned sleeping arrangements etc... you’re going to be severely unprepared.
First of all April is still quite cold in the mornings, after noon and at night - meaning you’ll need multiple layers of clothes for the changing conditions throughout the day. During this time of year the water temperatures of rivers/streams/lakes whatever are going to be COLD ! Not really a good idea to jump into a freezing lake in a cold day...
You’ll also find once you get out of the cities and into the more ‘rural’ areas of Japan you’ll have a hard time finding replacement parts like tubes/chains/tires whatever if you need them. It might be slightly easier than rural America but not easy by any stretch of the imagination and you could be in for a long walk. (Ask me how I know !)
Also once you get out of the city a few hours /hundred kms- depending on the season you’ll find that they’re basically uninhabited. For example if you go near a coastal sea side town which is usually bustling in the summer with thousands of tourists, in the off season (October ~ ) you’ll be hard pressed to find open restaurants / even hotels are closed etc...
I think you need to put more planning into appropriate clothing, sleeping arrangements, carrying 2-3 tubes, spare tire, chain, tools etc... + you’re going to need to charge your phone every few days.
Anyway if you make it through Osaka I’ll be more than happy to take you on a 2-3 day trip through the mountains Osaka / Nara /Kyoto /mini / Wakayama.
Great riding ! Good luck
