The initial run of your mountains is not bad, but (as Diamond has already pointed out), you don't have enough detail. In the tutorial you linked, notice how his shading is varied to show how the light is hitting the mountains. This variation in shading is what sells it (and as Diamond pointed out, putting more and finer strokes on there will make it look better precisely because they will give the impression of detail). One really quick thing to try is to switch the colors on the mountain ridgeline and rivers to get the rivers darker than the mountain ridgeline. If you're going for a purely pen-and-ink style, that might be helpful. If you're willing to make the mountain ridgeline lighter than the background, that might also help sell it as higher, but that's not normally an effect you can do with pen and ink.

Quick exercise: Think of a mountain range. Now think of a river. Most likely, the mountain that you thought of was all jaggedy angles and the river was smooth curves. While there are lots of rolling hills in the world, not too many rounded terrain areas get to be called mountains. Rivers wander back and forth in their courses as they wear away at the world unless they are heading down the jaggedy mountains, so they tend to swirl and curve. If you take those mountain strokes and draw through about every third swirl with a line (ideally on a different layer to get the comparison), I expect that you'll be much more pleased with the results. I would also say that the centerline of the mountains should be a lighter weight stroke so that it's easily distinguishable from the rivers, but that may be much less important if you reduce the swirls. The color change I mentioned earlier might also help.

And if you only spent eight hours on this learning opportunity (including new tooling and techniques), then you're far ahead of where I was way back when. Or maybe even where I am now...