If you spend a lot of time sitting at a desk you should get up once in a while and do some exercise.
Easier said than done - so set a reminder to yourself. For example, use Outlook or set the alarm on your cell phone. Good times are in the morning at around 9:30 and in the afternoon at around 3pm. At those times, force yourself to get up from your desk and go for a quick walk around the block, or several blocks if possible. About 10-15 minutes of brisk walking is ideal. Now, you won't always be free at precisely that time each day, and when that happens you need to use another hack. If that time of day isn't exactly right (maybe you are right in the middle of something) you should snooze the alarm for 15 minutes or 30 minutes and take your exercise break as soon as possible. DO NOT JUST TURN THE ALARM OFF. You are important, your need to stay healthy, so you must get your exercise for the day. Otherwise, studies show that you will become more prone to diseases such as heart disease and cancer.
I expect some will be thinking "but my boss will get upset if I take time to do that". Well - if your boss really will get upset about you taking a little time to care for yourself, you should set the boss straight. You are going to take care of your health first. If your health goes downhill your life will suck so you must make it your number one priority, which means it's ahead of your job. If this doesn't fly with your boss, you need to do whatever is needed to fix this. One option is for you to find an alternative job.