As a cold war state, Ukraine has tons and tons of bunkers, tunnel networks, and underground military facilities under its major cities. All likely packed with food, ammo, and other supplies for a long war.
Russia is going to have a hell of a time trying to clear them out without resorting to leveling whole cities with bunker-busting missiles and artillery.
And they can't do that because the West is watching too closely and wouldn't allow Afghan-War-style scorched earth tactics on Ukrainians.
They've really gotten themselves into a bad spot.