The principle that seems to work really well for endurance athletes can be summarized by; “Train on fat. Race on carbs”.
The fuel limitation to long duration (2+ hours), sustained, high output exercise will be the amount of glycogen (a form of stored glucose derived from carbs) the athlete has on board. By becoming more fat adapted the athlete will be able to conserve his or her very limited glycogen stores by using a higher percentage of fat as fuel at all intensities. This fat adaptation strategy is accomplished by the methods covered in these articles https://uphillathlete.com/?s=Fat.
However, during the big day(s) of your event you NEED to keep the available glucose levels in the blood topped up. Because, when they run low you will be forced to slow dramatically.
Steve, Mark and Scott had ample fat stores for the duration of the Slovak direct climb and were very well adapted to conserve glycogen. What they were not able to do, and no one can, was have enough stored glycogen to last 60 hours. By regularly topping up the carb fuel tank during the climb they could prevent a complete let down when the glycogen fuel tank ran dry.