You also have to take into account that just because Russian troops roll into or through a town doesnt mean they have "taken" it. Expect a good amount of guerrilla warfare, even from armed civilians, perhaps not intially but possibly prolonged, especially if the Russians intend to "occupy" these areas.
The Ukrainian military also has some effective weaponry at its disposal, its a matter of the fighting spirit of its soldiers how "easy" it is for Russian troops.
A large or prolonged number of Russian casualties surely will not be popular with the Russian populace or even the Oligarchs that currently support Putin.
