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.
I recommend getting up every hour or two for five to ten minutes, walk to the water fountain, stretch, whatever you can do to get moving. Personally, I walk at lunch, something I never did when I was obese. I find that walking gives me more energy and I don't feel as sluggish in the afternoon.
For those with office jobs, you should get a pedometer. This simple device will keep you moving as you will know how much activity you are doing each day just by looking at it. The American Heart Association recommends getting 10,000 steps a day. I've been getting that many steps in for over two years (minus one day of being sick), so I know this is a goal you can achieve.
I can't believe how long I used to sit without moving. My feet would be swollen and hurt. Another problem was that the only exercise I would do was walking a few steps to get my lunch. I'm so happy my habits improved as I feel better about myself and this is the best shape I've been in since college!