Nice start. A few suggestions:
--thinner lines for the main outline;
--lighter lines (I find two ways of doing this: either, reduce opacity from 100% to about 80%; or use a dark colour that matches the overall palette; in that case, a very dark brown)
--some of your lesser ridge lines look good, others less. I guess practice is the key to that, and, in my opinion, it is always useful to research. A general suggestion, though, try to make the lines longer, and more varied (they don't all have to come from the central ridge).
--maybe: less twisted shapes for the mountains (the twisted shapes you've done can be a good choice in some settings; especially if you are aiming for something that looks jagged and creepy)
--try to make the shading fade as it reaches the bottom of the mountains. My method is to lower the brush opacity to ~20% (or even 5-10%) and, rather than using brush strokes, just keep clicking until you build up the amount of 'ink.' Doing it this way allows you to make the colours darker in certain areas, and lighter in others. Aim to keep it darkest close to ridge lines (main and lesser lines)
--add some highlights: same as the shadows, but with a light colour. Again, aim to place it on the ridges, but on the sunny side of the mountains.
--I think a good idea with mountains, especially when not using some kind of preset mountain-shaped brush, is to try to make ranges of mountains, rather than single peaks. It tends to look more natural, and also serves to highlight the hand-drawn essence.
--it can sometimes help to change the colour of mountains in various places (eg greener near the base, pale or near-white at the tip).
--a trick I use to make mountains look more realistic is to include wisps of cloud around the peaks (a low-hardness brush and medium opacity, followed by the blur tool), and/or fog in the valleys.
--practice, practice, practice (deliberately copying or tracing someone else's mountains--for private purposes, of course--doesn't hurt)

still...for now:
THW