The short answer is that it's very simple once you overcome the steep learning curve, but most people give it up well before then.
My best advice is to spend two or three hours with an experienced builder on a vox program like Team Speak or better yet, Skype, for some one on one training. You'll be good to go very quickly and will learn the shortcuts that shorten your task from years to a few weeks.
Two important points for the long answer.
A new terrain engine is in alpha testing. Its development was announced as having begun in August of last year so hopefully the new game engine is very close to release. Current terrains are supposed to be forward compatible with the new engine, meaning what you develop now should work with the new engine. I think I can safely say that your bases, towns, strats and roads will be replaced.
The new terrain engine will require a new Terrain Editor. The current distance between elevation vertices is 1/2 mile, the same as it was when you were building terrains years ago. The new distance between vertices is 1/8 th mile or 660 feet, allowing much more detail, but meaning much more work. Fortunately, there have always been tools and shortcuts to make the task quicker.
