I have to limit my time just sitting. When I'm quilting a quilt, I set the timer on my phone for a 1/2 hour. I get up and walk around then. Usually I go downstairs, put some clothes in the washer or dryer and do something else for a bit. I often alternate sewing on one project with cutting out another project. My body seems to be happier with me that way.
I also try not to hunch over my machine when I'm working. Sitting up straight seems to help