I think it's just a function of working on it at a different scale than what you're exporting it to. Is there a reason you're working on it at such a large resolution? 20k square is roughly mural-sized; "good" printing resolution is around 300px per inch. So even a huge poster map (24"x36") only needs 7200px x 10800px. You mention that it's for a novel; if so, the maps are routinely printed at about 4x6 inches for a paperback, giving you 1200x1800px. So I'd suggest starting with a smaller size, which would have added benefits of not bogging down your computer (unless it's a lot more powerful than mine!) and being able to post it up at full size. Give yourself some ability to print largish (if it were me I'd go with 4800-6000k or so), but 20k is almost guaranteed to be overkill (and make all of your symbols look really really tiny for all practical purposes).

Unless you want a mural or tapestry for your wall, in which case I pronounce your mountains just fine