The render time of my second image was way too long, and I'll have to go back and optimise it before trying again. In the meantime, I have done my first big size Terragen render, deciding to do something a bit less time consuming. Just shy of 24 hours later I had a 4000x8000 picture that is quite simple, but I think I like it.
I have also made a few other scenes that I intend to render soon. My computer will probably be working continuously day and night now.