The areas where Russian troops head to are not only flat terrain but also completely devoid of forests.

The Ukrainian forces are practically confined to cities/towns outskirts perimeters because advancing/attacking/counter-attacking outside the buildings and civilian population proximity would dangerously expose them to air, missile & artillery attacks.

Thus maneuvering and logistics probably give Ukrainian top brass a massive headache. For fear of rapid destruction, not because they could be stretching lines too much, which could become a problem for the Russians, but I doubt it's a significant one at this stage.

Regardless of morale, fortifications, willingness to profit of the Russian reluctance to hurt civilians, Ukrainians find themselves reduced to point defense, in utter impossibility to coordinate and carry out large formations warfare. You simply can't win in these circumstances, if decisive external help doesn't arrive rather fast.

Guerilla war ? Maybe, but that kind of warfare comes only after you have lost the conventional, regular armies clash, one. Partisans combat at this point simply can't exist - flat land, no forests and minimal number of cities occupied. You can place a few road bombs but it really wouldn't matter.