The first 10 km and last km run on concrete alongside a fairly busy road, but the rest is almost entirely through forest and it's worth it: it leads to nice little summits with a view on the Fuji.

I barely crossed paths with anyone on the trail, just two people 6-8 km deep in the forest, so you should have it mostly to yourself and be comfortable with it. I wore my bear bell, though I'm not sure if bears actually roam the area.

A heads-up: the first few kilometres into the forest are not very well signed.

I originally found the route on AllTrails (https://www.alltrails.com/fr/randonnee/japan/yamanashi/lake-yamanaka-outer-rim-loop) but tweaked it to avoid starting next to the road. The trailhead is easily reachable by bus.