I set my timer for 30 min. I have a bottle of water in my sewing room. When the timer goes off, I grab the bottle, walk around for a few minutes, drink some water, go to the BR. Then sit down, set the timer again, and start sewing again.
Which foot do you use on the pedal? Try alternating feet. I do that. I have a Neez EEz that keeps my foot pedal from sliding around, and I put the non-sewing foot beside the other foot, so my back isn't 'skewed.' Also, if your chair is hard-edged, try a foam cushion on your chair so you're not cutting off circulation on that hard edge.