I looked at every button in the f-curve editor and couldn't figure out a way to make a linear animation curve continue on it's current trajectory beyond and before the two keys defining the angle of trajectory. It would be a nice feature to have so you could create longer animations by just placing two keys, changing to a linear curve and letting the path angle extend forward and negative. In 3DS max they call this "Out of Range Types" as seen in the screen capture.
Any chance this functionality is already in Terragen?
I'll add this to the issue tracker as a feature request.