In order to win, Chad and Fred will run the race at AeT. Since they ate the same diet and have performed the same work-outs, for simplicity sake let’s assume their AeTs are equal. They have full and equal glycogen and fat stores before the race begins. In summary, they run most of the race at the same AeT, and therefore at the same HR and speed. Aerobic metabolism supplies the ATP, and since they’re at AeT, by definition 50% is from carbs and 50% is from fat (they are at the crossover point). From a biochemical point of view, carbs and fats are equally efficient at producing ATP, per unit of oxygen: they both produce about 5 ATP per O2 molecule. Both Chad and Fred have unlimited fat stores. At the start line, Chad and Fred both have about 2000cal worth of glycogen.
Let’s say they burn 4800cal during the 6hr race (~800cal/hr@10min/mi), which is about average or so. Since they have the same AeT and are both running at their AeT, by definition they’re using the same amount of carbohydrates per hour (400cal), at least until Fred runs out. Fred, with 2000cal of glycogen, runs out of carbs at ~5hrs. Chad, in essence, has unlimited access to carbs during the race, thus preserving his glycogen stores. Overall, Chad was best able to spare carbohydrates during the race, which is perhaps the main factor that determines onset of fatigue. Note, however, that without carb supplementation Chad’s ability to spare carbs would still be equal to Fred’s, since they both have the same AeT. Chad wins the race because he could maintain his AeT, while Fred has to slow down around 5hrs because he can no longer maintain AeT, which by definition requires 50% carbs as a fuel source. If the race was say ~5hr long, they would have tied.
Carbs are always important on race day—ie, the carbs you already have within you. Carb supplementation, per se, may or may not be important on race day, depending on the distance (and intensity level). Chad’s supplementation would not have helped him win the race if it was say ~2-5hrs long—assuming they raced solely at AeT, which on average is the fastest one can race at these durations, and neurological factors are excluded. To maintain AeT beyond about 5hr, a carb fueling strategy is required. That’s not to say one can’t run way beyond ~5hr without carbs, but that they will be doing this at a pace below AeT. This underscores the importance of improving one’s AeT.
(I hope this reasoning isn’t too reductive to the point where it makes my argument false)
