I am using GIMP. I sketched on normal paper an overworld map, scanned it to my computer, and so far have finished using GIMP to outline all the borders, mountains, rivers, etc... basically just drew over all my sketched lines. Now I have a huge 5048x6458pixel map outline, black and white.

I am ready to texture it. I wan't it to kinda have a worn out look to it and I am interested in this look here:

But I don't know how to texture it like that in GIMP. Could anyone help me out here, maybe reference me to a tutorial that does just this, or provide me with instructions on how to do this?
If brushes are involved, I would rather not use any brushes someone else made already.

I already tried this tutorial:
But it does not look exactly how I want it to; when I did try it out, due the the extreamly large size of the image and when viewed at 100%, the bumps were huge and it just looked odd, fake, and ya know...just did not work.