Here are Part I and Part II.
First, a progress report: The surface and two levels are done. So is one sub-level. A further sub-level is in progress, and I'm almost done with another full level.
Copy Things,or, The World is Your Geomorph. If you like something on another map, copy it. Either use it as inspiration, or just use it.
I've found that copying is the sincerest form of not having to map stuff. I like those caves in a particular module? Copy them onto my map. Need a cool temple area and someone did a great one? Lift it. I literally copied a few maps, shrunk them down to match my page size, and physically pasted them on. It works fine. This does mean my maps can't ever really be published as-is, so I'd need to change it. But it's not meant for publishing, rather for gaming, and I'd really like to use those particular areas. Besides it's GURPS and I'm mapping on squares, so it would need to be re-mapped anyway.
It's the same, really, as sticking a published adventure's dungeon out on your hex map somewhere. Or using Gygax's Dungeon Geomorphs. I'm just sticking in stairs that say "leads to the dungeon entrance for
Corollary: You don't need to match scales if you go "off the map." If you do copy a small area, yes, scale must match the rest of the map. But a teleporter that dumps you outside the the Lost City or inside the Mines of Moria or something doesn't require matching scale - just switch maps. If a staircase goes down from your 5-per-inch 10' per square dungeon map to a 4-per-inch 5' per square dungeon's second level, just swap maps when it happens.
Corollary: You can use as much or as little as you need. Quite simply, steal what you need. Need a watery cave? Find a watery cave in an adventure and use that. Need a temple? Just copy the temple. You don't need to honor the original creator's complete vision because you're not here for that. You're speeding up your mapping. You can use the whole thing, of course, but you don't have to.
By the way, this includes Earth - just copy stuff from Earth. Your PCs haven't been there, either, so just copy those dark caves and strange temples and lost cathedrals. If they have been there, well, give their PC some strange knowledge - either explained by skills or by visitations from the spirits or something of that sort and let them use it.
Keep Mapping I've said this before, but the key is to keep drawing. Don't draw and erase, draw and draw and draw. Keep mapping. Set it aside and you're likely to be finished. Take the map out now and just draw a room or two. You'll thank yourself later.