Lines such as these are good for setting the perspective. Tighter lines suggest a more 'zoomed out' picture of the area.
I did something similar with the coastline of Mystical Era (see spoiler below).
Only differences really are that I didn't use the border function (I've never actually tried it before)...just grow selection, fill, shrink, delete. Your way seems more simple. Also, I used the feather tool instead of gaussian blur. I don't like the blur operations much.
You can see that I went one step more extreme and added a different colour under the lines here, just to really soften the gradient: