I don't know of any really good solution for just painting in arbitrary areas of rock. Unfortunately, one of the things you'll keep running into is weird looking shading - either too regular looking, or not lining up with the outline of the area - simply because the shape of a natural stone outcrop tends to be relatively complex. The solution you've used above is (in my opinion at least) a pretty good compromise though. Just means you have to let yourself be led a little by the shape of the rocks you've got, but hey, sculptors have been doing that for thousands of years, so it's a fine tradition.
A few tips and ideas to try to improve the look of your map even further:
Try painting the mask around your rocks with a textured brush - noise or even some more complex pattern. You want the two textures to blend with one another, while still having the illusion of a hard border between the particles of rock and particles of dirt or sand.
Make sure your light directions match up. The light on your rock texture looks to be from the top left. The rocks cast shadows that go down and left, as if the light were in the top right. This is pretty subtle, but the eye picks up on it as a subtle dissonance in the image as a whole.
You might try painting in some large scale form on your rock formations using a large noisy brush to daub simple black and white into low opacity multiply (shading) and color dodge (highlight) layers on opposite edges of the formations. This will help give them a little sense of depth and overall curvature.