Yeah, you just erase part of the stroke with the eraser tool. I usually use a soft eraser to do it before I blur it. I'm not very familiar with Gimp though as I use photoshop but I can't imagine stroking and blurring works much differently!

And anyway I doubt it'll be 150 years! I think you'll find you start to pick things up pretty quickly and each map becomes a bigger and bigger improvement. Keep all your older maps so you can compare in a few months time and I think you'll see what I mean.
Also it's worth doing any and all tutorials even if you don't like the styles or don't intend to make that type of map because I find there is always a little trick or technique, even if it's only one, that you can pick up and use in another map.