For several years I've been singing the praises of getting up out of your chair when working at a computer. Recent research shows (see previous posting) that morbidity and mortality greatly increase with sitting more than four hours per day, even with a good exercise routine.
What to do?
Get a treadmill and build yourself a simple desk over it, so that you can walk while you're at your computer. There are $5,000 systems on the market, but here's a way to do it for just $39.