You probably know that a triathlon of any length consists of a swim, bike and run, but to succeed at the half and full distance you'll have to learn a fourth discipline: nutrition. Unlike the sprint or Olympic distance triathlon, the half and full IM becomes about how fast you can swim, bike and run, and just as importantly, how you can take in enough calories to keep from bonking. Unless you are a pro, it will take at least 10 hours to complete an IRONMAN. In that time, it will be impossible for you to take in more calories than you'll burn.
The human body just isn't designed to take in more than several hundred calories per hour during intense exercise, while at the same time burning four of five times that amount. The trick is to figure out how your body reacts to prolonged stress and how to best compensate for this intense and long exercise without bonking. A successful IRONMAN triathlete has spent considerable time honing their nutrition regimen prior to a race. And as odd as it may sound, when you step up to the longer distance races be prepared to feel sick. Gastrointestinal discomfort is common in athletes participating in the IM distance.