Just read an article about how exercising before breakfast burns more fat. Article makes it sound like it's new news. Heard this for a long time. Problem is that people will just read the headlines and then start running on an empty stomach. It's true that when you run on an empty stomach, your body will use up any remaining glycogen (carb) stores and then begin to burning fat. Nothing wrong with that. However, what a runner has to ask himself/herself is...."What is my goal?"
If your main goal is weight loss, then great idea to get up early and go for that 3-5 mile run before breakfast and possibly burn more fat than if you had eaten first. But, if your goal is training for a half or full marathon, then your main focus should be on achieving a quality run. Running 10-20 miles on an empty stomach is not going to provide a quality run. When a runner comes to me to talk with me about a recent lackluster run, 90% of the time it boils down to improper fueling.
Yes, you can burn fat as a fuel source. When all the fat is gone, you can even burn protein (muscle), which is not good. But the thing to keep in mind is that the body's main fuel source and most efficient fuel source is glycogen (what carbs are turned into and stored as in the muscle). Fat can be an energy source, but it's not as efficient as glycogen.
Sometimes weight loss will happen when half marathon or full marathon training, but I don't encourage runners in the thinking that running long is going to cause weight loss. If you are training properly, your appetite will increase and as long as you're appropriately fueling based on your activity level, then you'll be fine. Most of the time, however, runners will over estimate their calorie burn and they'll eat a lot more than they need. My other concern with this article is there will be people who think.... "I'm going to run on an empty stomach and then I'm not going to refuel afterward either. That way I'll really get a good burn." Problem with that thinking is that when you drastically decrease your caloric intake and drastically increase your activity level, the brain will go into protective mode and actually latch on to and protect your body fat and burn muscle instead. Decreased muscle means less caloric burn, which can mean weight gain or a weight plateau. Not good.
So, long story not so short...if your goal is weight loss, then try running shorter distances before breakfast as a part of your weight loss goal, but if you're goal is a marathon....eat.