There is another way and that is to use Tools/Copy & Paste/Height. To do this:
Make an area of the desired elevation using the terrain brush. Then copy and paste height of a reasonably sized rectangular or square area. I have found this can be faster for doing large areas, but if you get carried away, or do it too quickly, expect a CTD (crash to desktop) as a result. So save often.
Now if you add a texture to the area you copy from, you can have a different texture for each pasted elevation by Copy & Paste/Height and Textures., so it makes visualizing elevation differences much easier. Once you are done terraforming and have smoothed out the terrain differences, just reset all these textures to the grid or whatever.
Also, Tools/Copy & Paste Height will also paste water. This is useful when you want to create large area of ocean or lakes. For an ocean, copy terrain at say -20M with water and you can quickly make large areas of water.