The United States defends Japan. The United States has nuclear weapons.

Knowing those two facts, we can conclude that Russia will not be invading Japan as long as it is protected by the United States. The calculus is very simple when nuclear weapons are involved.

Also, keep in mind that D-Day was the largest amphibious assault in history and all they had to do was cross the English Channel. Russia invading Hokkaido would be suicide, the US nuked Japan rather than try invading the homeland to end WW2 and we controlled every island surrounding Japan at the time.