If you have looked in on the Terragen Discussion section here lately you will see that Planetside Staff offered a detailed outline of how to optimize render times. So many things affect it that the obvious is overlooked while details are focused on. As it was with what I had previously done.
This then is the result of what Oshyan had figured out. I ran several test renders, changing or adding things, but left the Render settings alone.
I'll let you guess how long this took on this old machine -
A - One hour
B - Less than half an hour
C - Two hours
D - Less than 3 hours
I'll let you know after only three responses.