Thanks, a friend on Facebook eventually pointed me in that direction. I got that part figured out.

So, to keep updates current (in the thread at least, if not in the tutorial itself)

Select the 100 hardness tip and set the size in place of sized brushes.

And Export, not Save As, to save the brush file.

Also, the path (at least in Vista) for saving the brush is C:\Users\Owner\.gimp-2.8\brushes

I've gotten as far as making my 2 sets of mountains and a set of trees. I'll post any more updates if I find more out of date info as I progress!