What I think is important, that was not mentioned, is to build up your aerobic base to better take advantage of depleting the higher fat stores in your body.
When we run slow (our base aerobic level) we burn fat first and in greater amounts than carbohydrates (stored as glycogen in the muscles). The article mentions our body stores a lot more fat for energy use than glycogen, so why not teach our bodies to burn fat as a primary energy source. To do this we have to increase our base aerobic level.
For example, when I first started running my base aerobic level (conversational pace) was around 9:00/mile pace. This means at 9:00/mile pace and slower my body was using predominantly fat stores for energy use. Any faster and my body switched to using mostly the less available carbohydrate stores in my muscles (glycogen). As I continued my training my base aerobic pace improved to 8:00/mile pace. This means I can have longer “long” runs during training, further helping me develop endurance for long distance events. I can go 18+ miles at an 8:00 pace with less likelihood of hitting the “wall” (glycogen depletion) because my body has been primarily burning fat for energy. (Keep in mind the body always burns both fat and carbs for energy, just in different ratios). The more aerobically fit you are; your “slow” runs will use more fat than carbohydrates.
What does this mean for race day performance. When we run at race pace, usually 1:00-1:30 faster than our “slow” pace, our bodies burn closer to a 40/60 ratio of fat to carbohydrates (glycogen). Remember, speeds faster than our base aerobic level will always use more glycogen than fat. If our base aerobic level is slower, then at higher speeds, our bodies will burn a greater ratio than 40/60 (sometimes as low as 10/90! When we start panting and are short of breath). All this info. best serves runners looking to race an event and to not be content to just finish. For the rest of us, improving our body’s ability to burn fat first serves our training better (we can go on longer runs) which will help us become faster in the long run.
Dietary Fat and Endurance Athletes | Active.com.
This site also explains the role of fat and sugar (carbs. aka glycogen) and how it’s used as energy.
A non-scientific example!