Sitting is tied to mortality -- independent of physical exercise.

Joyce switched to a sit stand desk: http://pandawhale.com/convo/426/a-week-with-a-sit-stand-desk

So did Gina Trapani: http://smarterware.org/7102/how-and-why-i-switched-to-a-standing-desk

Sitting kills us faster: http://www.businessinsider.com/sitting-can-kill-you-2012-4

Sitting shortens our lives: http://www.nytimes.com/2012/04/29/sunday-review/stand-up-for-fitness.html

On average, reports Gretchen Reynolds of the New York Times, every hour of TV you watch cuts 22 minutes off your life (slightly less for women.) Men who don't watch TV live 1.8 years longer than men who do.

And it's not just TV. Sitting all day at work hurts you, too.

It makes you fat.

It makes you weak.

It makes you more likely to get sick.

How do we know? More studies.

Here is what happens when you sit: http://www.nytimes.com/2011/04/17/magazine/mag-17sitting-t.html

Electrical activity in the muscles drops — “the muscles go as silent as those of a dead horse,” [inactivity researcher Marc] Hamilton says — leading to a cascade of harmful metabolic effects. Your calorie-burning rate immediately plunges to about one per minute, a third of what it would be if you got up and walked. Insulin effectiveness drops within a single day, and the risk of developing Type 2 diabetes rises. So does the risk of being obese. The enzymes responsible for breaking down lipids and triglycerides — for “vacuuming up fat out of the bloodstream,” as Hamilton puts it — plunge, which in turn causes the levels of good (HDL) cholesterol to fall.

You can't counteract the harmful effects of sitting by exercising once in a while. If you exercise a lot but also sit around a lot, you'll still have a shorter lifespan than people who don't sit so much.

But here's the good news:

You don't have to start running marathons to offset all that sitting. You also don't have to get a treadmill at the office and sweat all day. You just have to get up every hour or two and walk around for a while. Or, alternatively, you just have to stand instead of sit.