I have begun to use a system I recently read about and it is working great for me. My wife helped by purchasing an inexpensive kitchen timer that I use. I set it for 55 minutes then sit at my desk and type until the timer rings. Then I go and set it for 10 minutes and lift weights or do some general chores about the house until the timer rings. So at least once an hour I am up and moving about doing some physical exercise.
I have found that when I return to my desk each time it is with a more optimist feeling. I am stretched and pumped up a bit with the pysical activity I have just finished and feel upbeat about returning to work.
Knowing I get a break in another 55 minutes I work steadily until the time ends. It is so much better than working for hours sitting at the desk getting tired and feeling sluggish and uncomfortable like I used to feel.