Neither Ukraine nor Russia seem to show a lack of manpower anytime soon. The issue for both sides are mostly material, which much depends on how aid Ukraine will get and how large Russia's weapon stockpiles actually are.

> What happens however if they push Russia up to their border? Does Russia continue firing from the other side? In which case does Ukraine then have to cross the border?

Ukraine has already been attacking military targets in Russia proper, so that's definitely something they'd continue to do if Russia refuses to give up. Sending troops in any significant number over the border seems unlikely tho.

But I doubt that would be an issue. If Russia would lose the war that hard, war support would probably be quite low in all classes of society.