Oooh, a topic I've spent waaaaay too much time thinking about. For me, sitting all day is bad for my body and worse for my mental well-being. It's a real problem, and no amount of hiking on the weekends and time in the gym really helped. For the record, I am in excellent shape. I exercise daily. My diet is impeccable. The problem IS long. stretches. of. just. sitting.
After much dithering, I bought one of those wheeled adjustable over-bed tables for about 50$. I also bought a cheap stationary bike (about 140$). So I can bike when writing, stand when writing, dance around if I'm working on something that only requires half a brain.
It really DOES make a difference. I write faster and better. I am so much happier and relaxed at the end of the day.
...To paraphrase Samantha in a Sex and the City episode, I'm riding mine right now...