I also find desk work and office work more tiring than physical labor where I would be moving around more.
I think part of the issue with office work, where you're mainly sitting for hours, staring at a computer and typing in things or even doing paperwork is just the mental tiredness and how boring it can be.
I have worked both in office and outside and I much prefer working outside, but currently am back in an office due to losing the other outside physical labor job.
The office job pays more, but I really hate just sitting at the computer for 10 hours a day, with just an hour break in between for lunch and of course I can get up to use the toilet when needed.
But just sitting around and not being outside is driving me nuts and I wish I could quit this office job and go to a job where I was working outside again.
Or even if I could just find a job where I was moving around more.
But I can't afford to just quit this job in the office right now, as the economy sucks right now and expenses are high.
I have a friend that thinks my job in the office is just so easy and not exhausting as he has told me that he wished he could get a job in an office, just sitting around at a computer all day and making more money.
He says it must just be very easy and not demanding, but office jobs including those working on the computer are mentally demanding more than physically demanding.