Both of the books I wrote, one of which includes two complete implementations of a programming language, were mostly written in sessions of less than an hour. Occassionally I get longer ones and very often they are much shorter.

Learning to task switch and suspend efficiently is also a really valuable skill that improves with practice. I have kids, so if I couldn't make progress while being interrupted, I'd never be able to do anything.

And I wonder how you do it. How are you able to suspend and resume work on a complex project like this? How are you able to stop thinking about it when scheduled time is up, and then pick it up the next day, without paying a huge time cost of loading yesterday's state back into your head?

Could you elaborate on this? I'm asking seriously - I myself wish I could do that. The way I work today, in irregular but long bursts of high focus, is somewhat effective, but doesn't lend itself to having a balance in life (work-life balance, but also "personal interests - life balance").

There is an apocryphal story of what Newton said when he was asked, "How did you come up with the theory of gravity?" replying: "By thinking about it all the time."