The good news, is that unlike short course racing, where the majority of the energy is going to be provided by carbohydrates (most of which are already stored in the body prior to the race), as the intensity decreases, the amount of calories from stored fat is going to increase while the amount of energy from carbs is going to decrease. For even very lean athletes, they are going to have at least 20,000Cal of stored energy in fat which is going to provide most of the fuel for race day.
However, as anyone who has completed a long or ultra distance event (or even a long training day) can tell you, eating is crucial and is going to make the difference between finishing the race and ending up in the med tent. So, why is taking in fuel going to be important if the body has so many stored calories? The reason is that although fat can be utilized as fuel, the body needs carbohydrates in order to use the calories in fat.
So, if an athlete needs to eat but cannot look to replace all the calories they burn during the event, where is the athlete to start? A good starting place for an athlete is going to be 300Cal/hr on the bike and 200Cal/hr on the run. Using long rides and runs, in addition to race simulations, an athlete can refine the number of calories they are going to be able to consume during race specific efforts and use this to build a plan that will get them through the finish line.
Also, as an athlete develops their fueling strategy, I believe they should avoid making a plan that is too intricate; if the athlete can handle what will be served on course, this greatly simplifies the logistics and also helps reduce the possibility of not having sufficient nutrition or having to overload their bike. The picture shows the bike of an athlete who plans to supply their own calories for an entire IM ride (plus a few extra calories, just in case). For example, in reading through some plans and race reports, I will see that an athlete would have a gel, sports drink, part of an energy bar and a piece of banana each hour. While it might seem like a good idea to have several sources for calories, in the end it adds the possibility of confusion on fueling during the race.
As a race goes beyond approximately three hours, nutrition is going to become a crucial element to race day success. Knowing that you will not be able to replenish all the calories that are burnt during an event and practicing your strategy in your training is going put you on the right track to getting to the finish line appropriately fueled.