Making Fractal lines in GIMP

[Cool as a Cucumber]

I'm looking for some sort of tool for GIMP, Photoshop, or PaintNET (that last one is unlikely) that will allow me to make a randomized line or perhaps even a randomized shape that looks like topography. I created a map in Fractal Mapper 8 (seen here) that utilizes these sorts of tools which was great for my concept, but actually making it look pretty is something the program is not very good at.

Ideally I want to make the borders glow (like this) as well, but I'm also going to need to put rivers on the map as well as make other modifications. I took a look at the GIMP tutorials, but none of them mentioned a randomized line tool like what I need.