Asia on a tandem

I've ridden the Tokaido route between Tokyo and Kyoto. It was largely a solid route and very convenient to use/follow.

http://www.japancycling.org/v2/

Another place to look could be the Japan Rapha Travel trip itinerary, I've not ridden any of the routes but I'm sure they'd be good.

https://www.rapha.cc/gb/en/shop/japan-randonn%C3%A9e/travel/T18086

Hi there, we based some of it on japancycling, but went off the route around the second week. Some of our highlights are not there: western shore of lake Biwa, Tsunoshima, and the coast northeast of Tsuruga. Make sure you go to the alps, it's absolutely wonderful. Kusatsu is another highlight I'd definitely recommend. Oh, and do the Shimanami Kaido.

Most of our navigation was on Osmand maps, and we used maps.me very occasionally to check for shops and whatever. Google maps was much more useful though. Buy a tourist SIM card at a Bic Camera if you need data, otherwise there's plenty of free wifi around.

Convenience shops are convenient but more expensive than supermarkets. Try to always plan a supermarket stop for lunch, food is cheap (for Japanese standards) and it's impersonal enough so you can sit down for a bit and charge your devices without anyone bothering you.

Campsites are fairly basic and sometimes far from shops, so plan ahead. We found some beautiful ones, just make sure they'll be open. Or not, just show up and pitch your tent, I find it very unlikely you'll be troubled. Alternatively sleep at michi-no-ekis, these are roadside stations that sell local produce and have wifi and toilets. They open late and close early (6pm ish), so you can pitch your tent somewhere unobtrusive without too much worry. Sometimes we slept on the floor or inside tables, if there was a lobby open.

Hope it helps, pm if you want to chat more.