It doesn't seem like a big deal to sit at my desk for 8 hours straight. In fact I've gone much longer only getting up from my chair to get food or go to the bathroom. But yesterday I worked that long! I never thought I'd need to get away from my desk. I mean, I worked so long I finished one audio book and got halfway through another.
Sure there's plenty of times I don't want to sit at my desk, but those are usually just about totally being in love with the idea of the couch. After all, I have a laptop there.
It seems weird to complain about working for 8 hours straight. I used to do it all the time. The difference I think was that that job was a lot of driving around. I spent at least half the time driving. Even on the day I mostly stayed in the office, I still buzzed around the office in the morning and still had to deliver a thing. And I had to print a lot of stuff which meant getting up to go to the printer.
Doing computer maintenance for 8 hours straight was "woah".