At the moment, I'm currently in that process with the co-author of my webcomic, Parmeshen--both for the comic and for a different project we're working on together. Since the comic started out as a play-by-post roleplay, there are a lot of holes and world building things that didn't make sense originally, but we didn't harp on it too much.
It's really entertaining to me to look at what we wrote and what we had in mind and try to edit it into making sense. We've had to re-write several scenes, add quite a few new ones, and reconsider things we'd taken for granted during the initial story. It's almost like going back and writing an entirely new story around the very basic framework of the original.
All this to say that sometimes people can use world building as an excuse to hold them back--"but I don't know the rules of magic and [insert famous writer here] said I can't start writing until I have them pinned down"--but it doesn't have to, and probably shouldn't. It's okay to start writing without knowing your entire world, as long as you know there are flaws and you're prepared to smooth things out in edits and rewrites.
I guess it goes back to that advice of finding your own writing process and being true to that. However you write is how you write, and as long as you're getting words on the page, you're doing great.