I think it changes with age. When I was quite young, 10 hour a day for days on end wasn't so hard to pull off. Remembering to sleep and shower and brush my teeth were harder. The catch was that a very high percentage of the code I'd write was either pure crap, or could have been done better in less time by writing another program to write that code. As I've gotten older, I've found that it's easier to spend a large number of hours *contemplating* code -- but hard to work other than in smaller bursts actually writing the code. The difference is that when I do write code, the hours are far less wasted.
I've taken this into account and so now my plan is, that when I reach 90, I'll just wake up in the morning and fart. My heavily customized Emacs will analyze the fart and translate it into C. "Oh, boy, I wrote another new OS kernel this morning!"
Well, ok, one of the two above paragraphs is true and not the other.