So far I have only created a small 300m x 300m area with fog limiting visibility. A map centred on this area can be seen here (in my existing project).
The terrain is generated by the same limitless terrain generator as in my existing project.
I have reached this point (version 20.2.10) in a couple of weeks by reading first Jos Dirksen's book "Learn Three.js" and then the threejs fundamentals pages. The latter were particularly important for discovering how to make the geometry of the ground using my existing terrain generator. Without that I was close to giving up the idea as being too difficult.
The next challenge is to change the area of the mesh as the user moves forward. Things coming into view must be added to the scene and those left behind must be removed. Temporarily the user is prevented from going beyond the edge of the 300m square area but there should be no edge.
20.2.16: Problem with camera movement corrected.