We can adapt to most things. That means we can make our body adapt to bed rotting all day or working out first thing in the morning. It just takes a consistent schedule. To paraphrase a running monkey from Bojack Horseman, "it's gets easier, but you gotta do it everyday. That's the hard part, but it does get easier."
Funny story! My kids were appalled that we were on a 1-mile hike to the creek last summer.
"ONE MILE!?"
Little shits were sore as hell walking out. Guess what? By the end of summer, a 2-mile round trip in the sand was nothing to them. Guess what else? We used to jog a mile, at the start of gym class in the 70s.
But you’re not doing nothing. Sitting uses your muscles. If you’re sitting all day then you’re overusing those muscles, which leads to muscle tension and spasms. Thus pain.
It’s like beating yourself in the head all day with a hammer and then wondering why you have a headache.