It would actually be easier than Georgia, I suspect. The big problem that Ukraine has is that, like most other ex-Soviet states, it let its military deteriorate in the 90s to the point of utter inefficiency (did you see the photos a govt guy just posted of what their BTR reserves look like, in response to a Facebook question as to why volunteers aren't getting vehicles?), but unlike them, it didn't get a wake-up call until now, like Russia itself got in Chechnya, or Georgia got in Ossetia and Abkhazia. So now they have to recover and learn very quickly. There's a lot of enthusiasm on the troop level, but logistics is in shambles, their officers seem to have a poor grasp of tactics (like e.g. ordering an artillery unit to stay in one place while firing... needless to say, they get fucked by counter-battery fire, and the reason why we know about this story is because there were survivors), and their generals don't understand that grand plans they make bear little in common with reality. This, again, is a lot like Russia was during the first conflict in Chechnya, but that was an easier opponent, and consequences of defeat were not as far reaching.
What's going for Ukraine is that their population reserves are bigger, and they retained a larger arsenal as part of the Soviet legacy. Also, the fact that a significant part of Soviet military industry was in Ukraine, so they have experience manufacturing the things they need.
Either way, I think that the only reason why they can still fight effectively, even with large casualties, is because Russian involvement is still undercover. It became noticeably less so over the last week, what with armored columns openly crossing the border (but still with removed flags) etc, and notice how the situation that was so dire for the rebels suddenly became so dire for the Ukrainian troops. If Russia were to go all in, openly, throwing all units that it already converged at the border, I don't think Ukraine stands a chance without outside help.
How long can Russia occupy Ukraine, now, is a different question. That area has a long history of guerrilla warfare against occupiers of all kinds, including Soviets back in WW2 days. And there's a strong resolve to resist among the populace today. An occupying force might win in the field, but find itself facing bullets from every window in the cities at night.