I am having some trouble figuring out how to make bogs in GIMP, and have them not look terrible.

Here is how I made bogs in a hand-drawn map, later edited in GIMP:
Name:  Bog1.jpg
Views: 800
Size:  22.5 KB

And here are the same bogs made directly in GIMP, on a map with a parchment effect. In this, the bogs were made by overlapping and editing the "coffee stain" feature.
Name:  Bog2.jpg
Views: 669
Size:  10.6 KB

Now, I am not 100% sold on either of these, but they seemed to work well enough. Now I want to figure out how to make bogs fit into this map:
Click image for larger version. 

Name:	Abred1.jpg 
Views:	614 
Size:	3.03 MB 
ID:	45341

I can't find a useful tutorial for it here, and my google-fu is not turning up anything helpful either. The most useful thing I can find is a thread Jaxilon started a while back. It's not a tut, but it at least provides something to potentially emulate.