If Russia occupies Ukraine, it will become another Afghanistan for them, Chechnya, and the other places they have subdued are tiny, to occupy and control all of the Ukraine would require a long term commitment of 50% of there armed forces and even then they will be under constant harassment from IED's, and guerrillas.

I'm form Canada, which has the second largest Ukraine ethnic population in the world and a lot of ex-military personal from here are going volentarrily without support from the Canadian government, apparently enough to form their own battalion. Same thing from the USA, ex/experienced military serviceman, some special forces. Each of these guys is probably more capable than 20 Russian, 18 year old conscripts.

Put these guys in charge of some of the hardware that Canada and the US are sending into the country and you have some pretty formidable opposition. Once again, most of these guys are hardened veterans from Afghanistan and Syria, so they know their way around an RPG and other weapons or are capable of taking control and using captured Russian Tanks and Armoured Vehicles. If you get 20,000 of these guys in there, I know a big stretch, that would pretty much negate the entire Russian portion of the army made up of Conscripts that don't want to be there in the first place.