So, what memory do you have? A minimum would be 32gigs I'd say. And how large is your population? If you try to cover 10k with trees 2m apart, you'd likely get in trouble with meagre memory, especially with heavy (MB) trees. So it's wise to place the population area smart, the area starting in front of the camera, so no or a few trees will be instanced behind the camera. And perhaps use lowpoly trees for really distant woods, or less dense. Like this very simple setup.
It is also good to restart Terragen after a lot of trials of terrain or so, so the cache will be emptied.
