Finishing a book does strange things to your brain.
I tend to write the ends of books fast and prefer, where possible, to revise fairly fast too (though there are multiple passes at this).
For the first draft the endgame speed comes from the part where I finally see the shape of the ending, how everything is going to come together and I just want to get it down on the page. It's like the weight of the whole story gathers in my brain and kind of pushes (which will sound weird to anyone who doesn't write...the closest analogy I have, having seen two babies born, is that you get to a point where you've just got to push and get through to the end and you really don't have any choice in the matter). I often know the very final scene early in a book but, being a pantser, have little clue about how I'll end up at that point. For my revisions, I need to hold the whole story in my head and look at it from various angles and stitch together more firmly. And having that same sense of the weight of the whole book needs a certain kind of focus that works best if I can do it fast while I can sort of sense all the connections.
My process has changed over time and with changing from writing 240 page to 400 page books. My first drafts are shorter and that makes the revision that much more work. I wrote 90 new pages in two weeks for this revision plus made multiple changes to every single one of the original 338 or so I had at the end of draft one. Yesterday I wrote for close to ten hours to finish the second draft. And then I wrote for a few more hours on another wip because sometimes the writing just can't come to a full stop immediately.
And today I have post book brain, where I'm coming off the adrenalin jag of all that focus and pushing myself to write. My brain has a space in it where the book used to be and it feels like everything else is shifting around to settle back to where it belongs. It's hard to concentrate on any one thing for more than thirty minutes or so. I want to nap but can't quite relax enough. I want to write something new but also don't want to let this story go entirely because I know in another few weeks I'll have comments from my agent and crit buddies and will need to plunge back into it for round three. Plus there's the inevitable "oh my god is anyone going to like this or is it completely terrible" writer paranoia while I wait for said comments *g*.
It's a combination of satisfaction of being done (and of having actually finished a book this year after struggling to write at times and writing less than I would usually), of being tired, of not entirely wanting to relax and take a few days off because the writing is working again and writer paranoia imagines that it could just vamoose once more and of not being sure what to work on next anyway. It's frustration with feeling this way even though I know it happens every time I finish a draft. It's not helped right now by the fact that my exercise options are somewhat hamstrung by my dodgy shoulder. But tomorrow I shall do something to move the butt or the whole process is just going to take that much longer.
It's hard to explain how something that's almost entirely a mental exercise can leave you feeling so steamrollered.
I wonder if this is what bands feel like when they finish a tour? Probably fifty times worse because writers at least, don't have the crowd high to come down from as well (distracts self with somewhat amusing mental image of the writer in glamourous faded trackies and t-shirt typing on stage while the audience screams).
Anyway, that's as coherent as I get today and for the non-writers, don't feel bad when your writer buddies zone out, stumble around and forget minor details like the outside world when they've just finished a book, it's not you, it's them and post book brain.