The only passes with roads through a very few river valleys through the caucasion mountains to the north of Ukraine are very long, winding and not really very helpful for moving armies through. I don't think that the geography would make this very easy for Russia. The Ukraine would not be a push over either, it is a very big country, with very big armed forces.
Russia's armed forces are also heavily committed in too many other places as well. The Russian's are very unlikely to want a war on this scale. The death toll would be millions and the cost would be thousands of billions. Russia just can't afford that. Even after the war, the counts of maintaining enough troops in Ukraine to control a very angry population, for who knows how many years, would be totally crazy.
Russia has miscalculated, the Dooma is expecting the water shortage to cost Russia 900 billion dollars to get it sorted, if a way can be found to get it sorted. Invading Crimea at all, was a very bad idea for Russia and the ongoing situation is not getting any better, any time soon. Russia has built a very long road bridge and a very long rail bridge between Russia and Crimea across the sea. It could be the most expensive bridges which have been built anywhere in the world.
Ukraine has some very large and well equiped armed forces, with plenty of investment in modern weapons and their people are well motivated to fight. Russia is just hoping that one day, the ukrainians will get fed up and agree to a peace deal. Ukraine is very big, but not big enough to beat Russia. Russia is a military super power, but the cost of suceeding invading the whole of the Ukraine is not affordable, so stalemate!