I did the reverse of this as part of a coast to coast ride. My route was about 190 km and used a lot of cycling roads, so away from the worst of the traffic. I'll post up the GPX track when I get home today.
I've done the flat and easy (boring?) half of that route twice just during the last week: it's a cyclists' trunk route in and out of Tokyo. When are you planning to ride, and have you ridden in Japan before?
Thanks this is a really interesting report, although now I started to think about Coast to coast as a challenge, but maybe that for next time ! I lived in Tokyo some years back and regularly travel to Japan but never rode seriously on the roads there, is there something special I should consider? I will plan to ride in early October this year when hopefully the humidity is less but the weather still good