Long, slow distance—carried out by weekly long runs—is key to your endurance base. It teaches your body to burn fat and increases aerobic capacity.
Long runs should be done once a week and slooowly.
Long runs build aerobic capacity and train your body to burn fat. Your interval training and tempo runs will help with aerobic capacity as well, but only the long run will effectively train your body to burn fat.
Here's why you want this.
You only have a certain amount of sugar in your muscles and your blood. You can burn all of it up, and quite quickly at that. When you do, you'll feel what's called "hitting the wall." You simply can't get the energy up to keep going. It's an awful feeling.
Fat cannot be burned fast enough to sustain a fast run. But you can burn some fat! And the more fat you burn, the less sugar you burn!
You will not run out of fat. Even if your body fat percentage is 4% and you weight 130 pounds, you still have over 5 pounds of fat on your body, and 5 pounds of fat will carry you roughly 180 miles.
So make your body good at burning fat, thus conserving your much more rapidly burning sugar. Do this with a long run done slooowly!
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