This may sound crazy coming from a dedicated runner, but cardio is overrated for calorie burning. Cardio does burn calories, and it is important for a healthy heart, but if you really want to burn fat ALL DAY LONG you need to do weight training. The afterburn from cardio lasts a couple hours, the afterburn from weight training lasts a couple days. The best form of cardio for weight loss is HIIT. Also, if you weight train and do cardio in the same workout session, do your weights first and burn 50% more calories. Also, on a low-carb diet there is no need to exercise on an empty stomach. Drink a protein shake before your workout to spare your muscles from being burned off as fuel.