Very good attempt Mouse, It is the very same way i learned how to draw mountains. Though i pretty fast change my "Mouse"

to a small tablet, which makes a big difference. Then i just spammed whole canvas with mountains for a long time
About Gimp and Krita. Both are ok but have their disadvantages. Gimp for me is totally unintuitive software. You can learn it and use it with success, but if you jump from Photoshop or any other software to Gimp, it is a nightmare. There are always a set of shortcuts that are used the same way in many programms, but gimp breaks that rule and turns everything upside down. Except quite good variety of filters it lacks fundametal options to make your workflow easy and smooth.
Krita is much more complex then Gimp. It is much easier to work with. Has normal sets of shortcuts and everything lays where is should, also has fundamental features which Gimp lacks, but as you might encounter the brush engine is harder to understand and use. The biggest advanatge of Krita is that it is design for drawing purposes only (and animation right now). You can find there nice tools do draw with stabilizer and mirror brush. Gimp is an overall raster graphic that is focused on drawing and photo editing. You won't find there so many options for drawing.
Both are fine considering they are free, but in my opinion if you go strictly for drawing, Krita is a way to go.