Being able to run, and run hard for a solid 20 minutes is great and all, but it's been proven that your body will burn fat more easily when put under constant change.
With that being said, let me introduce you to my favourite kind of cardio: interval running.
For those of you who may not be familiar with interval running, it is when you run for a short burst of time, followed by a even shorter recovery period, and then repeated multiple times (this can also be done on an elliptical trainer, stationary bike, ect).
For an example, see the interval running routine below:
When working out this way you push your body to it's limits by running at a high intensity, then you trick your body into thinking it's cooling down and just when your heart rate slows a bit you crank it back up there and push yourself back into high gear which results in more calories burned. You can also increase the incline or adjust your speed to give yourself more of a challenge.
Give it a try and push yourself to the limit, and get ready to sweat.