Years ago, I read The War of Art. I highly recommend it. The author, Steven Pressfield, was on the Huberman podcast explaining how he writes a book and overcomes the Resistance.
 
The whole episode is great, but I thought he dropped one golden nugget in particular: he only writes for 2 hours a day. 
 
You don't have to quit your job or rearrange your whole life to write a book. You just have to commit 1-2 hours a day. 
 
Like Steven, this has been
my approach with The Human Diet. For one, that’s about all the time I can afford to spend writing. But also, after one or two hours, both Steven and I experience drastically diminishing returns—any more time doesn’t add much progress.
 
Bonus: For
me, I apply this concept across my life. For example, many hours of working out doesn't provide much (if any) more results than just one hour.