If you have you mountain already, switch to the materials, as if you're going to paint on a single trail. Have it around 30 to 40 on the slider.
Don't make it a straight line, wiggle it a bit as you go so it looks natural, and try and go over, bumps, uneven terrain etc.
Good distributions to use when you paint with them:
Small Rocks: set to about 55/60 on the slider with a small brush size and dab them around.
Twigs: Set to around 55 on the slider and dab them on the trail and to the sides.
Bushes, and Small Trees/Small Trees Dry can go on the edges at around 60 on the slider.
Note: On the slider is Density, not scale.
I'll show you an example.