I would suggest changing you focus from 'staying fit at work' to 'get fit in general.' Work is not making you out of shape. Once you make up your mind to get fit, it isn't difficult.
1. Log everything the goes in your mouth. Everything. Gum. Coffee. Water. Cough drops. Vitamins. Get a very accurate count on calories. Alter the foods you eat to drop the calories to a normal level.
2. Ate lunch at your desk, then go for a walk for your lunch break. If you can't walk during lunch, walk for an hour before or after work. After awhile, you can change the walking to jogging. Take it slow to start.
3. Make the food you eat from ingredients/cut out processed stuff.
After a while, maybe start lifting weights. Squat, deadlift, pull ups, push ups, rows. Lot of compound exercises with weights. Start with an olympic bar and add weight when it is manageable. Look up from 5x5 program. They are easy to follow for people new to lifting.
I've dropped from 300 pounds to 190 in the last year. 3 days of lifting and 3 days of lift cardio per week, alternating weights and cardio. Rest 1 day a week.