The first draft of Debris of Shadows Book II just crossed the 300 page (75k words) mark... and, according to my outline, still has at least another 100 to go. Coffee is my friend.
The difficulty is fourfold: First, it's a sequel, and it's honestly going to be one of those where you have to read the first book to really know what is going on. Serialized novels have become more popular these days (Thank you, George R.R. Martin) but I'm sure I'll receive some flack for that. Oh well.
The second is that the story goes back and forth between The Sage in NorMec, and Matthew's adventures in WesMec, and then ties the two up at the end, which will lead us into the final book. (Yes, there will only be three.) Multiple plotlines have to be equally interesting. I've read books where the A plot was all I cared about, and I just wound up skipping through the B filler in annoyance. All the characters involved have to move the story along, the lazy bastards.
The third is re-purposing. Matthew's continuing story springs from another novel I had gotten about 200 pages into over a period of about fifteen years, which had many of the same themes. I was able to use about 3/4 of the plot, but it had to be completely rewritten. The art is in disguising the art, or something like that, which means part of the job is making sure no one sees the seams.
And fourth, as John Lennon said, "Life is what happens when you're busy making other plans."