A recent study suggests that even exercise won't undo the harms:

[The researchers] found that the more hours the men and women sat every day, the greater their chance of dying prematurely. Those people who sat more than eight hours a day — which other studies have found is about the amount that a typical American sits — had a 15 percent greater risk of dying during the study’s three-year follow-up period than people who sat for fewer than four hours a day. That increased risk held true in the Australian study even if the people sitting eight hours a day spent at least part of that day exercising.