Too much sitting is especially risky for women! and...exercise doesn't erase the effect of sitting at the office all day. The American Cancer Society study found that:
-Women who reported more than six hours per day of sitting were 37 percent more likely to die during the time period studied than those who sat fewer than 3 hours a day.

-Men who sat more than 6 hours a day were 18 percent more likely to die than those who sat fewer than 3 hours per day.

-Women and men who both sat more and were less physical were 94 percent and 48 percent more likely, respectively, to die compared with those who reported sitting the least and being most active.

The association remained virtually unchanged after adjusting for physical activity level. Associations were stronger for cardiovascular disease mortality than for cancer mortality. The researchers concluded that public health messages and guidelines should include reducing time spent sitting in addition to promoting physical activity.

Bottom line, walk at least 5 mins per hour. (that will be the next study to see if short breaks from sitting help lower mortality rates.)

Source: American Cancer Society, July 22, 2010
http://pressroom.cancer.org/index.php?s=43&item=257