I feel your pain. Literally. I've become knitting-obsessed in the last couple of weeks, and frequently knit for two or three or four hours at a time. (I'm slow, too :shrug:) My neck is sore, my back hurts, and my triceps feel like I've had a workout!
Thanks everyone for the good suggestions. It is important to get up and move around once in a while, and I really need to work on my slouchiness!