Just had this cross my FB page. http://www.burnthefatblog.com/archives/2010/10/are_you_an_active_couch_potato.php
Bottom line is long,uninterrupted stretches of sitting does not do a body good even if one exercises regularly. I have to admit I'm luckier than most, my pay-the-rent work, although mostly tied to a computer screen allows me to sit, stand, dance, bounce, or whatever else I can think of. However, when I start to sew, I can sit for hours and not realize where the time has gone. One tip is to find an timer (I've found them at my version of the dollar store). I'll set it for 15 minutes as a reminder that I need to get up and stretch.