Same as always: getting to the point where you can believe in the book's world as a viable reality. I don't mean a "realistic" one – just one that's carried by its own momentum. Once you get past that point of critical velocity or whatever, the whole project flies – but it's a fucker to get there.
What, if anything, would you do differently if you were starting the book again?
Once you've written a book and look back at the process, it's like the build-up to a goal. You wouldn't change anything, even the missed passes and accidental ricochets, because they led to what turned out to be the thing.