What they're prepared for is a "strategic strike" and/or a limited scale invasion which is a part of a larger conflict in Europe. Combined arms and fast pace, but not a hybrid war like in Crimea or a separate war where Russia is only engaged in Finland. The Finnish Army reformed their doctrine quite recently [1] with the goal of ensuring adequate performance with lesser troops (war time strength has been reduced significantly in the 2000s).
[1] video with Eng subtitles: https://www.youtube.com/watch?v=2crAx8kibis
Russia is capable of dealing with long drawn out battles on home turf. They have not displayed that great ability to invade. To be real, neither has really the US either.
You can't underestimate an opponent fighting on home turf for their own independence against an opponent, where the soldiers don't really see the reason why they are invading in the first place.
Also remember for whatever tech or equipment the Russians have. They have really shitty democratic traditions. They don't let their men work independently. History has shown again and again that top heavy armies are ineffective.
It is how Israel beat 3x bigger Arab armies, even when the Arabs had better tech.
Russian and American forces successfully invaded German-occupied territory and Germany in the second world war.