It's also handy to know that right-clicking while in the brush tool will bring up sliders for brush radius and hardness, which is quicker for big changes to the brush than using [ ] or going up to the tool settings bar.

Regarding your question concerning mountain ranges, it helps to know why such ranges form. Mountains often form at the edges of tectonic plates, where one plate slides under another or two plates are crashing into each other and the earth piles up at the junction. The Sierra Nevada and Andes ranges are examples of the former, and the Himalayas are an example of the latter. I'm not sure how other inland mountains like the Rockies and Ural Mountains formed, being so far inland from the edge of their respective plates.

Mountains also form as a result of volcanic activity, which typically occurs near the edge of plates, but can also happen in the middle of a plate, like Hawai'i.

Anyway, since the edges of landmasses often coincide with the edges of tectonic plates, it is not uncommon for mountain ranges to mimic the coasts.