I have a The North Face ad hanging on the wall of my cubicle at work that says "Sit for Your Job, Run for Your Life." Only in the last 100 years would you see a slogan like that. "Sit for Your Job." 100 years ago, nobody sat for their job. People worked in plants or mines or farms. Very few people worked in offices. In fact, the only one I can think of off the top of my head had the surname "Scrooge."
A 2010 study in the American Journal of Epidemiology found that men and women who sat for more than six hours per day were respectively 18 and 37 percent more likely to die during the study than those who sat for fewer than three hours per day. Surprisingly, this number was completely unrelated to the amount of time subjects spent exercising.*
So you need to combat that statistic any way you can! You'll never "undo" the damage that all that sitting does, but you can increase your odds of a longer life with extra training time. There's nothing like a long run on a Saturday morning to torch all those extra calories that you couldn't burn sitting at your desk all week.
Personally, I'm going to start pushing the HR department to invest in some treadmill desks. Eight hours of sitting? Try eight hours of walking!
... but I'm not holding my breath.
*Alpa Patel et al., "Leisure Time Spent Sitting in Relation to Total Mortality in a Prospective Cohort of U.S. Adults", American Journal of Epidemiology, 2010, 172 (4), 419-429)
(See also, Which Comes First, Cardio or Weights?)