I cycled much of the northern Japan Sea coast last year. Compared to other parts of Japan, there is not so much to see. I wouldn't particularly recommend it.
As for Toukyou to Niigata, there is a very good cycling road (30+km) along the Tone River from Isesaki (in Gunma) through Maebashi (east of Takasaki) up to Shibukawa in the north. Route 17 from there is by far the easiest way to go as far as Jouetsu and there is often (but not always) a decent shoulder. You will go thru some tunnels so make sure you turn on your lights.
Try writing the Maebashi City Hall for a copy of their cycling road map:
2-12-1, Ote-Machi, Maebashi City, Gunma 371-8601 Japan