I've added 3000 words simply by going back and filling in holes. When I write a book, I push forward, hardly ever going backward. In fact, it was Rule #1 when I returned to writing, which I'm only now beginning to loosen up.
In the course of writing, I discover things about the characters and plot that need to be fleshed out. Just the process of going back and doing that will probably add an extra 5000 words before I'm done.
I used to think of a finished first draft as a complete story, that just needed to be burnished. Now I think of it as a framework, where I go back and add things and cut things and move things (slightly, not too much). I'm not trying to make the writing polished, but trying to get the story fleshed out. This needs to be here, that needs to be there. Figuring out the timelines.
The final draft, the editing and copy-editing is when I smooth all that out.
I'm probably going to make a day trip to John Day, to get some landscape description down. I was going to stay for a couple of days, but I don't think that's necessary.
I've written two of the Sherm chapters, and have two more I want to write. Only then can I sit down and do the re-write. So I may go past my May 1 deadline by a couple of days.
It's important I get this right. I have a big publisher willing to look at it, and that's an opening that doesn't come along very often (especially since I don't have an agent.) This editor actually sought me out originally, which I think is pretty rare. I figure the he'll probably give me more than once chance to produce a publishable thriller, but I shouldn't waste my opportunities.