The way you phrase this makes it sound like you have to work harder to burn more fat as you get more fit. This is not true. Say your cardio workout consists of running at N% of your max heart rate for 1 hr, covering a distance D. The more fit you become, the greater distance you will cover running at the same N% of your max heart rate. Since the number of calories you burn is proportional to the distance you run (work done), as you get fitter you will burn more calories without needing to exert additional effort. Cardio is a good way to lose weight, esp in combination w/strength training.