This is a really good article discussing just how much daily exercise a person needs to do in order to “offset” the damage done by how much time they spend sitting during the rest of the day.
Actually, the main message of the article is that we can’t really “undo” the nasty effects of sitting for eight hours a day. One study is cited as showing that 60 to 75 minutes of planned daily exercise might be enough, but not necessarily enough to bring about any additional health benefits. But the main point the article wants to make is that a good rule of thumb for health is to simply “sit less, move more”—no matter what that movement is. Which I can definitely get on board with.