Here's something I've been fiddling with lately.
This free asset:
contains several mountainous terrains that are pre-textured.
I drag one of the low-res terrains into a scene and adjust the terrain size and height.
Increasing the terrain size along with lowering the height makes those sharp mountain peaks turn into nice rolling hills. It's worth looking at each one to see how it will match with your existing terrain
Then I swap out the 4 textures on the outer terrain with several from my original terrain. The terrains are already textured which gives a nice start that you can work from.
From there you can adjust anything you want. This process takes about 10 minutes and I think the results are nice for the time saved.
Before and after with a few quick adjustments
Looks fairly natural even with no sculpting or painting adjustments yet.
Before and after on a hole that was close to the edge of the terrain.
Fix the texturing, smooth a few peaks, add some trees and I think it will turn out ok.