According to biomechanics scientist, Katy Bowman, sitting per se isn’t so bad as much as how much you sit in the same position. The issue (why it raises mortaility rates irrespective of exercise) has to do with restricting blood flow, which leads to plaque build up. Bowman’s advice is to minimize sitting, use a variety of positions, esp. those in hunter-gather societies.
See this article for more: http://www.katysays.com/your-position-in-life/
Plus, her book is boss.
Hope this helps!