Ukraine's military is completely hollow. They maybe have 10,000 actual front line capable troops - the rest are interior police and general rabble. Yanukovych was possibly even working in concert with Russia to make sure Ukraine had no meaningful defensive capabilities.
Ukraine could not even use the fraction of their armor that is actually serviceable in any kind of conventional formations since the Russian air forces would quickly gain air dominance and savage anything the Ukraine could assemble in the open.
Russia's military is also not nearly as capable as this chart makes it appear. Maybe 10% of Russia's military is 1st world capable, but they'd still quickly overcome anything the Ukrainians could throw at them.
The Ukraine would have to fight a guerrilla/partisan type conflict which surrendered land in exchange for harassing and bloodying Russian troops and pro-Russian militias. Much like the Russians themselves have done in the past against enemies advancing against them through the Ukraine.
The question, should Russia actually invade, is do the Ukrainians have the stomach to fight a prolonged guerrilla type campaign that saps the Russian will. Since Russia would probably only seize and attempt to hold the heavily ethnic Russian regions, it might be very difficult for the Ukraine to do much of anything other than try to get the West to admit them into the EU, etc.
The Russians do not completely have to mobilize their entire army into the Ukraine. Use Spetnaz, gain complete air dominance, and eastern Ukraine is theirs. All could be done in 24 hours.