Honest question, just because you can complete a half with no calories does that mean everyone can? I mean everyone's different. People have different weights, which impacts calorie burn rates. People sweat at different rates. Not everyone burns the same ratio of fat:glucose/carbs:protein when working out. Hell there are professional marathoners who don't even drink a sip of water during a marathon. I drank 40 oz on a 17 mile run last night.
The basic guidelines of burning 100 calories per mile run, having 2,000 calories of glycogen stored in your body, etc all are just that, guidelines. They aren't steadfast rules