(Qualifier : Obviously a good diet and a regular exercise program will provide a significantly greater health benefit than doing a poor job at either, but keep reading and find out why even for regular runners, sitting can do real damage.)
The attached link was published in this past month's Runner's World. I've actually been waiting to see something like this published for the past few years. If for nothing else, this article provides the ammunition that I need to combat those scoffers at work that make fun of how I try to stand up at lunch (particularly easy to do at most Subways and Chick-fil-As) or rise from my chair in meetings about every 30 minutes to stretch out and take a walk around the room.
(Get article here: Sitting is the new smoking - RW)
I won't try to summarize the entire article, it's much too interesting and you need to read it for yourself. However, in short, everything from increased risks of cancers, heart disease, diabetes, blood clots, on and on, are the result of what the article refers to as our "Car-commuting, desk-bound, TV-watching lifestyle." And similar to smoking, even if you are a runner, this pattern of behavior is not good for you, especially over 50 hours / week.
As I look back on my own life, I feel fortunate. Athletics kept us on the move as a kid, a walking campus and a full schedule left little time outside of the classroom to remain seated in college. My occupation has required long hours on my feet, and my hobbies are mostly mobile (even my guitar playing is spent jumping around the room in a 'WWBD' sort of way - What Would Bruce Do?).
So read the article and consider your own activities. Do try and park a little farther away. Do try and take the stairs every once in a while. Do try and break up the long hours at the computer with mini-walks. Heck, do even try and eat a Subway while standing up. You'll love it. Promise. Your legs and your health will thank you.
Bottom line; I guess it is true what the commercial says, and my parents preach, a body in motion tends to stay in motion!