'rather old', 'turning 50'. Geez! Sounds like you're still young with plenty in the tank. Just like me!

I can't offer any assistance about Japan sorry albeit to say I'm keeping an eye on this as I hope to do something similar next year. I was considering a guided trip as I would be travelling solo, or hire local guides to show me around but I'm not sure if that's a good option.

Glittering Mud by Stephen Gray

https://m.youtube.com/watch?v=1D0iHhLaNSU

I just ran it through my Pixel Song Search and it picked it up almost instantly.