Lots of great advice in these comments, including this one. I've also made a very conscious effort to keep my work from intruding on my personal life too much. Without that separation, one can burn out very quickly.

Also, I'm in agreement about the 2-3 hours of "golden" development, with the rest of the day for thinking, planning, or just doing something else. Sometimes I'm inclined to do 2 such sessions in one day, but a) I always make sure that the second session does not interrupt my sleep schedule, b) that I allow for plenty of down-time in-between, and c) that I don't do it for too many consecutive days.