If you work on the computer for several hours a day, this will make your life feel less like hell. But you actually have to stick with it.
Set a timer to go off every half hour. When it goes off, get up and stretch. Maybe do some jumping jacks or hula hoop for the duration of one song. Pick something easy.
If you work at an office, scope out somewhere private. Maybe a stairwell or bathroom. Stretch. Jog in place. Do some jumping jacks.
This isn't about burning calories. Your body isn't adapted to sitting still for hours with no break.
This will also improve the quality of your work because you'll come back to your tasks with a fresh mind.