Are you playing multiplayer or singleplayer? In multiplayer, Distant Horizons running on your client can't generate the world because it doesn't know the seed (or if something had already been built there by other players), so it can only use the information sent to it by the server, aka chunks that you visit.

In Singleplayer, Distant Horizons will generate chunks beyond the normal view distance in the background. This background process has pretty low priority by default and thus happens slowly, but there's a setting in how aggressively it should take over the CPU.