I have managed to come up with the following directions for cycling from Nagano to Mount Fuji by talking to both local and International cyclists. My goal is to make recommendations for safer roads with shorter tunnels. If you think you can improve on my directions, please do contact me so I can make adjustments.
I am not sure which road is best from the border of Nagano, but at least you want to get off Highway 20 as soon as possible. A nice diversion would be to stop at the Suntory Hakushu Whisky Distillery, but remember that riding a bicycle while under the influence of alcohol is illegal.
Follow road 12 for quite some time. I am sure there are places along the Kamanashi River where it would be wonderful to ride, but I have found that the existing paths often switch from one side of the river to the opposite side. If you don’t mind this inconvenience, riding along the river might be a good option.
If you need a place to spend the night, why not stay at my Airbnb property? In addition to being able to provide you with local tourist information, I can also help you find any services you might need such as a nearby shop for outfitting cycling enthusiasts. Furthermore, there is a great fruit and vegetable shop just a 1-minute walk from my guest house, a large supermarket just a 3-minute walk from my guest house, and many other services you might need that are within walking distance.