"After two weeks, I was able to stand for about four hours a day, but I still needed to take multiple breaks."
I work as a teacher. I don't sit down much in a typical working day, so I found that sentence strange to say the least. Then I realised that I don't stand still ever in the classroom. I'm effectively walking around my patch of savannah all day like our ancestors did, occasionally sitting or squatting down to talk to students on their work.
Perhaps it is the lack of movement that is the problem?