You must be looking at pre-WW2 borders. Large areas were ceded to USSR in 1940: https://en.wikipedia.org/wiki/Moscow_Peace_Treaty (and again with some more in 1944: https://en.wikipedia.org/wiki/Moscow_Armistice)
I think Finland is on eggshells here...
Isn't Estonia already part of NATO? Granted the Finnish border is a lot longer, but in terms of straight-line distance to St Petersburg it would be nothing new.