In addition to the "just keep trying" comments, I think it's also a thing that improves with age.

When I was younger, I used to be able to work "harder" but less sustainability, like a blue star that burns bright but supernovas pretty soon. Over time, I've gotten more like a red dwarf -- better at burning low, slow, and sputtery but sustainably. 1 hour per day adds up to a lot if you do it every day! You can read a 600 page book in like two weeks at that rate (and that's how I read books). :D