Helsinki is less than one hundred miles away from the Russian border. I suppose it's possible that a modern day Russian would make yesterday's mistake of wasting manpower in the forest meat grinders rather than focusing on the population center in the South, but I wouldn't count on it. The analogy would be the United States invading Canada. Sure, it's a big country, but 90% of the population lives within one hundred miles of the American border. A fat lot of good holding onto the Northwest Territories will do you when the American flag flies over Toronto, Ottawa, and Vancouver.
In any case, you got way ahead of yourself. Putin isn't going to invade Finland, Poland, or the Baltic States. The former is a member of the EU and the latter are members of NATO.