Research affirms this notion, and a large study involving over 100,000 adults found that those sitting for up to 6 hours a day had up to a 40% greater risk for death in the next 15 years as compared to those who sat for less than 3 hours a day. Surprisingly, these differences were found independent of whether someone exercises!
My recommendation is to avoid sitting as much as possible. If you have to sit at work, take frequent standing breaks whenever your schedule allows. Taking short standing breaks throughout the day is much more beneficial to your body than sitting for 6-8 hours and then going to the gym to exercise for 30 minutes. Not only will you be improving your joints and muscles, but you will actually increase your brain power by standing. More on this when I talk about standing work stations...
References:
http://aje.oxfordjournals.org/content/172/4/419.abstract
http://chriskresser.com/how-sitting-too-much-is-making-us-sick-and-fat-and-what-to-do-about-it