Being sedentary in an office setting can be detrimental to one's overall health. Keeping the body even lightly active over the course of the day will keep one's metabolism active, limit fat accrual and keep one's energy levels up.
Here are some tips on how to get physical - office-style:
Get up and walk around every hour from your desk.
Walk to co-workers to engage with them; don't call them.
Take a brisk 15-minute walk after lunch.
Limit the amount of heavy carbs (i.e. starches) that you consume during the day to avoid lethargy.