Cardio is nothing more than a less effective form of exercise compared to weight training (which can elevate the heart rate just fine). To a trained athlete, its fat burning effects are virtually non-existent.
The average weight loss after running a marathon is about 4 pounds. At least 3 pounds of that is water. That leaves a loss of one pound, at least half of which is muscle. That means it would require running 26 miles to lose a few ounces of fat. So how much do you expect to lose from an additional 20 minutes on the treadmill?
If you want to lose fat, continue your current weight training program and take in less calories.
Aerobics are the big lie.