Note that for the hills, I have very small bits of shadows with lot's of "smudge" so that there are a lot less "lines" of shadow and just slight hints. Also, anywhere that the lines end, either on mountains (note the left and right top mountains where the line just ends), you want to continue pulling down the color with the smudge tool.
One you have all the inner body of the mountains smudged down, switch over to the outline layer and smudge down a bit of the outlines for any lines which are hanging loose. You may want to lower the rate a bit or if using a tablet, lighten up on the pressure. For these lines, don't just pull straight down in the same direction, but begin pulling down in the same direction as the original line and then "curl" the line a bit as the smudging begins to "fade".
After a few more touch-ups, here is the final with all the smudging done
Optionally, if you feel like it, you can select the outline layer and reduce the opacity to make the outlines much fainter. In this case, I went down to around 30% or so:
I hope this helps anyone who would like some nice ISO mountains, but is a bit hesitant to give it a try. Please remember to rate the thread and post any comments, questions, or examples of your work.