Russia has only occupied a small amount of territory. They more territory they occupy, the more difficult logistics will be (though it will probably help when the weather gets warmer).

If their troops are hungry, given their undisciplined nature, they'll start going off on their own and foraging for food. And when they start fanning out to do that, their foraging parties will become very vulnerable to Ukrainian guerilla attacks.

Sort of what happened to the British in New Jersey in the winter/spring of 1777. They weren't supplied from home. They had to forage for supplies. And Washington smartly started sending out large bands of skirmishers go after those foraging parties. In the end, that raiding forced the British to pull back to New York because they were spread too thin.

Warfare has changed through the years, but the supply aspect of it has not.