The big thing is, Russia's strongest asset is the fact that it has around 45% of the world's functional nuclear weapons. If you take those into account, then any invasion of Russia would be doomed to fail as they have the option to flip over the table at any moment. If you don't, then Russia would be beyond screwed due to the way that modern warfare works. The Russian Air Force would be blasted to pieces the moment it touched the sky and then drones could grind down Russia's excellent AA systems. From that point, the Russian army would be torn apart by NATO airpower no matter how far they retreated as they wouldn't be able to mount an organized defense. In the long run, it would of course be a technical loss for NATO as they get worn down in an endless guerrilla war, but they would win the conventional phase of the war if they used even a small percentage of their assets.