At first, all exercise is anaerobic. Once you run out of the stored energy (glycogen?) your body needs oxygen (among other things) to produce more. Running is referred to as aerobic because you use the anaerobic up quickly.
Combining sprints with jogging in between mixes the best of both worlds. You'll see HIIT, interval training, and possibly fartlecks to those of you who enjoy cross country or distance in track
As for the sprints, just sprint for 20/30 seconds and then jog for 45 seconds. Make sure the sprint is fast and the jog is slow enough so that you can recover but you're still making modest progress. Don't sprint too fast or too long because you want to last the whole 20 minutes, kind of like how some inexperienced boxers come out swinging in the first couple of rounds. That might not be the best for you so you'll have to take a look around the internet to figure out what will be the best. Remember to stay hydrated with something that has carbs in it (maybe dilute some gatorade) and a little protein if you don't mind the taste. I don't necessarily mean a protein shake because some of that stuff messes with the stomach during exercise, but take a look around at GNC if you are really serious.
Since I'm computer inclined, I overlayed beep noises in an audio file. I dragged my music files onto the track above it and every time there's a beep, I either start sprinting or stop sprinting. That way I can have it beep halfway through a song or whatever.
It's a marathon, not a sprint. Okay so maybe that isn't the best...but you get the picture!