It's probably one of the best places to run in Helsinki as the quiet dirt paths are great for running even when its raining, and the trees in forest provide protection from wind and sun. There are also lots of hills available, so it's not too easy by any means.
The only downside is that the marathon course consists of 8 loops, so it might get a little boring. On the other side it provides a good opportunity to see other runners. Many of them are going to run shorter distances, but 42.2K is the longest official distance available.
I know there's also an ultra 100K race in the same area in August. I think it's 55 times about 1.8K loop or something like that, so I think I'll let that one pass.