To be careful is a good idea. Try doing the following exercises while at your desk, to ensure fitness if when you sit all day.
stand straight next to your desk or a table. Remove your shoes for ankle movement. Pull your heels off the floor, but keep your toes on the ground so that you are on tiptoe. Bring your heel back to the ground. Repeat.
sit on your chair with your back straight. Pull your knees towards your desk, so that your toes are raised a few inches from the ground. Hold for some time and bring your feet back to its position. Repeat with one leg at a time.