Guerrilla war is possible only in countries where is no quite big opposing minority. If to talk about Ukraine, then we can talk about 3 regions at north-west, i.e. Galicia - region near Polish border, there are living less than 10% of Russians. In all other regions are living about 25-40% of Russians and that means 25-40% of eyes everywhere, ears everywhere and any Russian understand Ukrainian language. In such conditions any guerrilla war will be very short, because Russians are after 8 years of pressure of Ukrainian nationalists, so they will watch closely around.
All East of Ukraine is most populated with Russians. Central Ukraine is steppes - there are nowhere to hide. And only some western regions near borders with Poland and Slovakia are somehow hills and small smooth mountains (almost without forest, because of barbarian lumber harvesting). There is no ground for long lasting resistance.