Walking is a great way to exercise because it's safe, easy and cheap. And you don't need to set a brisk pace to reap the benefits. A study by the Mayo Clinic showed that leaner people were more likely than heavy people to get up and move around throughout the day, even if they weren't necessarily exercising. The difference in calories burned per day between the two types of people was about 350 calories. If you tend to be more sedentary, this information might motivate you to find ways to be more active throughout the day.
For instance, why sit down to talk on the phone when you could be burning calories and toning leg muscles? Use your speakerphone, or a headset, so that you can move freely around the office during long conference calls. If you get writer's block in the middle of preparing the budget report, get up and pace until your mind clears. Drink a lot of water during the day so you have to get up and go to the bathroom at least once an hour.
Any excuse you can think of to get yourself out of the chair and on your feet is a good excuse -- as long as it's work-related, of course!