Russia is essentially a country you only want to invade on a rental not an ownership basis. The Crimean War was poorly run and a mess but they only tried to take the peninsula and didn't try to go any further so, that might well be the model.
Having said that, If the US Air Force and Navy aviation as well as NATO airpower were involved in this conflict Ukraine would have reclaimed all its territory by now. Ukraine is resourceful and awesomely resolute...they lack the sophistication to use airpower like we would, and it would take a while to get them trained up to do so.