mobcity zkore gives good advice. You can also: sit on an exercise (Swiss) ball, kneel with padding under your knees, kneel on one knee with the other foot planted in front (so that everything is approximately ninety degrees from each other), and alternate with your other leg every now and again - a delicious hip flexor and quad stretch. And also mix all of the above suggestions, so you're incorporating regular movement. You can even spend some time in your chair.
If the damage you mention is back pain, when you do decide to sit on a chair, you want a neutral pelvic tilt, so make sure you are sitting on the edge of a cushion, or have the front of the chair seat lower than the back. I'm not sure why chairs continue to be designed with horizontal seats, which facilitate pelvic tucking. I also like to support my back with a cushion behind my back.
I've read that even just standing up and sitting down every 30 minutes can do wonders and add years to your life, and it's hard to talk yourself out of just standing up for a second, so think of doing this as an absolute minimum.