I am not, by any stretch of the imagination, an athlete. I have however been active enough to have participated, and continue to participate, in some endurance sports, particularly endurance cycling. I'm not a runner. The thing is, I've participated in and seen enough of these events to see how most people are constantly in search of carbs to keep themselves fueled along the way. Many still argue that "carb loading" before an event can optimize your performance by increasing the amount of glycogen stored in the body that can later be used as energy. See this and this.
The thing that the concept of carb loading misses is that a typical marathon runner can only store about 1,600 calories in the muscles and about 400 in the liver, and this is probably a high estimate. Eating more carbs can’t jam more glycogen into muscles once they’ve reached capacity, the resulting glucose in the blood will only get turned into fat. You can use nearly all the liver glycogen but muscle glycogen can only be used by the specific muscles that store it so your leg muscles will only utilize the glycogen that they store, or the liver glycogen. So, let’s say out of the total 2,000 you could store, your legs and brain can only use about 1,700 calories worth. (An extremely well-trained athlete, doing very carefully timed carb loading and training might actually be able to store and use up to 3,000 calories, but I'm guessing this will not be the average reader.)
The number generally referenced for calories burned in a marathon is about 100 calories per mile so you’re going to need around 2,600 calories. If you aren’t adapted to burning fat (which you won’t be if you eat those plates of spaghetti to carb load and down the energy drinks and gels during an event) you are going to have to make up the difference somewhere. So you need to have the gels, drinks, and beans to make up the difference. Unfortunately, if you try to consume those too fast you have a problem with the concentration of sugar (osmolarity) is the stomach. You can only tolerate so much or you are going to get sick. From what I’ve read, you can only absorb about 100 calories every 45 minutes to an hour during a marathon. It doesn't do much good for the calorie deficit I refer to above, to consume carbs at the very end of the marathon, and your ability to digest decreases as the marathon goes on, so with a median marathon time of around 4:45, you are only going to be able to consume about 400 to 500 calories. (If you run faster than that, you're going to need more calories in less time and can absorb even fewer calorie during the event.
That means you are only going to have about 2,200 to 2,300 calories at the very most to play with and you are going to need about 2,600. This is when you hit the wall — you simply run out of accessible energy. If you could get to the extreme 3,000 calories available to an elite athlete (with very well-timed carb consumption and training) you should be able to do a marathon without hitting the wall, and not having to cram down fast carbs via gels, energy drinks, etc. But for most of us, that's not going to work. We simply can't get enough fuel. A good article on this subject is here.
If you look at endurance cycling, it's even worse. Most typically burn about 800 calories per hour cycling with a 15 MPH average speed. So for a century ride (100 miles) that's about 5,300 calories in about 6 to 7 hours. Hence cyclists tend to load up on bananas, granola, peanut butter and the like at SAG stops.
But there is a better (at least different) way. Carbs and fat are the two primary energy sources our bodies can use. Unfortunately, if we regularly consume a high level of carbohydrates (say 150 to 200 gms/day or more) the insulin that must be secreted to process those carbohydrates, also locks up fat stores. For many of us, those fat stores are not easily accessible until our bodies are forced to learn to use fat. This is a process that takes a week or more while minimizing carbohydrate consumption. At most a typical person can store, and access for running, about 1700 calories, as mentioned above. But even a very lean (10% body fat) male at 175 lbs would have over 60,000 calories stored as fat. A more typical, healthy weight individual would have more like 100,000 calories or more of stored fat. If you consistently consume high levels of carbohydrate and continue to consume those throughout an event, you can't access that energy stored in fat because the insulin in the bloodstream blocks its release. I compare this to a tanker truck hauling gasoline that runs out of fuel (diesel) on the highway. It's loaded with fuel that it can't use.
Contrast this to someone who is adapted to burning fat and ketones. (This is what happens if you limit carbohydrate consumption to less than 50 - 75 gms/day for about a month and is called a ketogenic diet.) Even an average, lean individual now has access to nearly 100,000 calories. They should be able to complete a marathon without taking in any additional calories. Think about it, no carefully timed carb loading, no trying to consume enough energy sources during an event without getting sick or hitting the wall.
This is true if the exercise remains below the anaerobic threshold since both fat or glycogen/glucose can be burned aerobically. If the exercise is above the anaerobic threshold however, glucose or glycogen are necessary. (This might be the case with a sprinter or weight lifter.) Most endurance sports participants would remain below the anaerobic threshold for most of the time so this wouldn’t be an issue with endurance running, cycling, or triathlons.
If you really want to geek out on this, here’s an article about how Dr. Peter Attia experimented on himself with exercise of various levels of exertion both in and out of ketosis. Some other good information on the topic of nutrition and endurance sports performance are in this paper and this video by Dr. Jeff Volek.
Finally, I recommend this book, also by Dr. Volek and Dr. Phinney on this topic and the success they've had in transitioning endurance athletes to a ketogenic diet approach. (Note that I receive a small fee for customers who purchase on Amazon through this link but it doesn't affect your cost at all.)
I've experienced some of these same benefits. For example, I can spontaneously go a day or more without eating and not feeling weakness or lack of energy. In fact, when fasting, I experience a great deal of energy. About a year after I started this approach to nutrition, I decided to start cycling again after a multi-year hiatus and ended up doing a century ride that year. I ate a high fat bacon, eggs, sausage breakfast (no toast) and then did the ride in about 6-1/2 hours, and consumed no real carbs along the way. I only drank sugar free energy drinks to maintain electrolytes and had a couple handfuls of macadamia nuts but never felt that I really needed them, and never felt any lack of energy. Similarly, I did a 1/2 marathon one afternoon (walking at a fairly fast pace -- a 2:41 time) completely fasted. I had no breakfast. I consumed nothing before, during, or after the event. Again, I never felt any lack of energy. I plan to do a century ride in the next year or so completely fasted. I'm not expecting to have any difficulty doing so, and know of others who do this routinely.
So while it's possible to get enough energy for endurance sports through carbohydrates and carb loading. I think there's a better option and I for one plan to continue to take advantage of it.