Ukraine has practically nothing to fight with. Their military has been gutted, they gave up their nuke warheads under the ridiculous belief they'd be protected by the West, their conventional stockpiles have been largely destroyed (thanks to Obama and Lugar), etc.
Further, in the ethnic Russian east they'd have part of the their own population working against them.
The Ukraine would have to cede land and try to bloody the Russians with guerilla/partisan type activity. They probably couldn't really fight until/unless Russia advanced far enough West into solid ethnic Ukranian territory.
Ukraine's best bet is probably to scream loudly about losing the Crimea, prepare to lose at least some further territory in the East of the country, and then use that as an excuse to guilt the West into allowing them into the EU and perhaps getting provisional NATO membership. In the end, some of these solid ethnic Russia areas probably need to go anyway in order for the Ukranian population to have a true majority of people who identify with the Ukraine rather than having so many sympathies with Russia.
This could be a blessing in disguise for the Ukraine if they play their poor hand well. This Russian aggression has probably pushed most actual ethnic Ukrainians into a full embrace of the West (even more than it already was). They lose territory, but by doing so they lose a large chunk of ethnic Russians who were making it difficult for Ukraine to have a distinct Ukranian identity and created election results that prevented them from fully moving away from Russia. And these actions may give Ukraine the leverage to get the West to more fully embrace them and allow them to integrate in the Western community of nations. Russia may gain a little territory here, but if the Ukraine completely moves away from them and tilts decidedly West - Russia still loses in the big picture.
Ethnic Ukrainians are physically indistinguishable from ethnic Russians, and most are fluent in Russian. Just as Iraqi Sunni insurgents had no problem striking in Shiite areas, Ukrainians will have no problem attacking in majority ethnic Russian areas.