A person can have 50,000 calories in storage as shown in the calculation above, but this doesn't mean that the metabolism of fat to glucose (gluconeogenesis) is not time or energy consuming. Thus, during a long distance event with constant work, there just isn't enough time to keep blood glucose levels up. The brain only functions off of blood glucose.. when it drops.. one bonks. This is why it is important to consume simple disaccharides or starch during and prior to the event, because they are quickly absorbed and feed right into the Krebs cycle in order to provide energy. So, everyone is right =)