I've done work I could do 100 hrs a week without burning out. Not saying I'd be happy doing it, but I wouldn't burn out. Moderately physical, some moving around, some sitting, a little variety, hard thinking not more than a minute or two an hour, interacting directly with the people who need me to do what I'm doing? I can keep that up a long time. 12 hours like that leaves me feeling less worn out, drained, and generally shitty than sitting indoors programming all day for 8ish hours.