It's actually best to travel to Imatra by car if you want to explore the area. Even Imatra itself is so stretched out that you'd be happy to have wheels.
Imatra is not so far from Russia as you can see from the various signposts.
You can take a bus from Helsinki all the way to Imatra. The fastest connection is 4 hours 40 min. but usually it takes around 5 hours and quarter.
From Helsinki it takes between 2,5 and 3 hours. The train is more expensive than the bus.