When you normally think of Cardio you probably think long slow distance on the treadmill/bike/cross trainer. Sitting on the same slow pace for 30-45min. What does this actually achieve? Not much at all....
If you are looking to burn fat and increase your work capacity at the same time within a short duration then start thinking more towards a conditioning finisher. A finisher is a short burst of high intensity work.
This page contains examples of different 'finisher' workouts. Completed at the end of your workout to add to your conditioning and help burning fat.