Strategically, Russia has to do whatever is necessary to maintain its Black Sea assets. The Ukraine will have to surrender the Crimea in exchange for a Russian OK for it to join NATO and the EU.
I suspect their pain threshold is not what it used to be. The Soviets cried uncle after losing 13K dead in Afghanistan. If the Ukrainians are prepared to take the pain of inflicting tens of thousands of dead on the Russians, odds are that the Russians will pull back. It depends on what the Ukrainians are prepared to sacrifice.