When I graduated college, I've had jobs that required sitting all day at the computer. I would sit hours and hours getting my work done without any movement at all. At lunch, I would hardly exercise at all. It's no wonder that I eventually got very obese.
There is nothing wrong with working an office job if you remember to get up and move every once in a while. If you just sit still for too long, not only are you getting little or no exercise, but you put yourself at risk for getting cancer, heart disease, diabetes and obesity.