It seems to me that the human body stores plenty of glycogen to run a 5K, 10K, or even a half-marathon. A person will never become depleted running these distances. Since depletion does not occur, does carbo-loading have ANY effect?
Going a step further, why is the Long Run important for races like 5K? I thought LSD was designed to deplete your glycogen stores so that the body responds by storing a greater amount to prevent depletion in the future. Since you will never deplete in a 5K, why do you need to boost the total amount of glycogen your body can store?