If you feel really crunched for time or 20 minutes of exercise seems like too much, you do have another option: intense interval training. Here's a seven minute workout from the New York Times:
The trendy new interval workouts are called high intensity interval training, or HIIT. This type of exercise means doing an intense interval of exercise followed by a rest interval. Research on the benefits of this exercise includes improved cardiovascular fitness and blood glucose control.
Though HIIT can be beneficial, as of now there is not consistent evidence that HIIT is superior to longer periods of moderate exercise. So, while I would still recommend 20 minutes or more of moderate exercise, those who feel that's too long or are really on a time crunch could benefit from HIIT training.
Read more about the potential benefits of HIIT Training in this article from NPR.
The best takeaway from the above article and from my own experience is: the best exercise is one you relatively enjoy and can stay committed to.
If you're interested in trying out some HIIT, here a a couple you could try: