How many times have you heard people talk about making sure you are in the "fat burning zone" or the "cardio zone" when you exercise? There are even modes on elliptical and bikes designed for these special zones! What has been assumed is that you have to exercise and low intensity levels in order to burn fat. Lets pick apart this myth, shall we?
First off, there is some truth to it. How many calories you burn during a given workout is directly related to intensity level. Furthermore, fat is the main source of energy during low intensity workouts. That is where people got the idea that they should exercise at low intensity to burn more fat.
Just for your information, when people do talk about fat burning and cardio zones, they are referring to heart rate. You are in the fat burning zone when you're heart rate is below 70% of your max. Then as you exercise consistently above 70% of your max, your body starts using carbohydrates as the primary source of fuel. This kind of exercise burns lots of calories.
So what's the conclusion? Real weight loss and fat loss comes from expending more calories than you consume. Therefore, you should concentrate more on burning calories than working out at a low intensity. Unless you have a large amount of time, I would recommend just kicking it into gear for a good, intense 45 minutes workout. Or better yet, interval workouts get your body to work in both zones. So mix it up, push yourself, and see the results.