When marathon runners train for an upcoming race, do they run the twenty-six miles right away? No, they train for weeks, months ahead of time, running shorter distances, thereby training their body for the ultimate test.
Do bodybuilders just do bench presses and that's it? Nope. They train various muscle groups, even switching those muscle groups so one set doesn't get overused.
Do farmers plant the same crop over and over again? Not a chance. They may plant a certain crop for a few summers or winters in a row, but they rotate after each harvest so give the soil the chance to replenish its nutrients.
Your brain is also a muscle, one that needs to be worked, challenged, and even rotated among various tasks. Ever been in an endless array of meetings? Afterwards, if you're not used to it, your brain feels stained, and can even have a "fried" feeling. But once you've been doing it awhile, that strained feeling grows less and less.
When I work for long hours on a novel--whether editing or writing the first draft--and I finally complete whatever stage the book is in, I always take a break from writing and read. Sometimes I don't even write anything for a few weeks. I call it "recharging my batteries." Brain batteries, in other words.
As you complete the first draft of your book or feel like your brain is strained from hours of writing, step back and do something else--exercise another part of your brain. Even work on a short story or another story in a different genre or a new set of characters. Take a break and read a little more than usual. Then, once you feel recharged . . . go forth and conquer!