While you shouldn't give up on your home or gym exercise routine, you can certainly supplement it with exercises done at your desk (and, on those extra-long workdays, it's much better than doing nothing.)
Here are a few aerobic tricks to try during your next break between tasks:
1.) Glance at the wall clock and rip off a minute's worth of jumping jacks. If you're a beginner, try the low-impact version (raise your right arm and tap your left toe to the side while keeping your right foot on the floor; alternate sides)
2.) Do a football-like drill of running in place for 60 seconds. Get those knees up! (Beginners, march in place.)
3.) Simulate jumping rope for a minute: Hop on alternate feet, or on both feet at once. An easier version is to simulate the arm motion of turning a rope, while alternately tapping the toes of each leg in front.
4.) While seated, pump both arms over your head for 30 seconds, then rapidly tap your feet on the floor, football-drill style, for 30 seconds. Repeat 3-5 times.
5.) If you can step into a vacant office or conference room, shadow box for a minute or two. Or just walk around the room as fast as you can. Or do walk-lunges in your office or a vacant room. (You could also amuse your co-workers by doing these in the hall; remember Monty Python's "Ministry of Silly Walks" comedy routine?). Set your PDA to beep you into action.
No conference room? Take to the stairs -- two at a time if you need a harder workout!
Do this 5-7 times a day.