Now, if we replace "eat glucose" with "eat carbohydrates", then the statement is still not quite accurate. Experienced marathon runners (and other endurance athletes) will not consume carbs before the run because doing so would spike insulin and effectively shut down breakdown of fatty acids in the liver. Liver can more or less either convert glucose to glycogen and fat, or break down fat, and is not great at doing both simultaneously. Maintaining fat metabolism is hugely important when going the distance since it's the largest energy reserve available in your body (50,000+ calories vs <2000 calories as glucose).
Marathon runners consume moderate amounts of carbohydrates DURING exercise when insulin response is blunted. The idea here is to maintain glucose levels and avoid going into hypoglycemic state (low blood sugar). Ingesting moderate amounts of simple and complex carbs during exercise spares glycogen without necessarily spiking blood glucose to where liver has to regulate toxicity. This way you can keep on going longer.
Just a quick note, and your explanation is fantastic otherwise: this isn't quite correct. The (human) liver doesn't use fat directly for gluconeogenesis. It's just not efficient for humans: We wouldn't gain enough energy from it. The liver uses fat indirectly through glycerol (which is a product of fatty acid catabolism), but it's important to draw a distinction here because this is why ketosis occurs - if the liver could efficiently break down fat directly, ketosis would never happen at all! Most tissues would much rather use glucose directly if it's an option.
While muscle breakdown occurs in small amounts normally it's mostly a last resort that occurs after an extended period of starvation. Which makes sense: You wouldn't want to break down the thing (skeletal muscle) you need to get more food except as a last resort. The amino acids involved in more typical gluconeogenesis largely come from other sources.
I also believe endurance or extreme endurance activities are more for your enjoyment than fitness. They take their toll on the body's joints, organs and regulatory systems. Moderation is good advice in lots of things.
A good heuristic is that if you bought it, or it was bought for you, it's got too much refined carbohydrate in it and the serving size is too large. This is because to consumers value=(volume*taste)/dollar
I'm not familiar with the effort required for mountain climbing, but eating a varied meal beforehand would help. Personally, a peanut butter sandwich, banana, half an avocado (depending on the size of the avocado), tall glass of milk, and celery with hummus will prepare me well for the following few hours. Adjust this for yourself as needed.
A healthy balance between fat, fiber, carbs, and protein is what you're looking for. I can't give any any hard numbers but a good macro-nutrient balance is the first step to a healthy diet, with the second step being fresh food.
Energy gels are usually high-energy but low-everything-else, which is probably what the parent is referring to ("eat glucose"). This is to supplement blood-sugar levels without affecting the digestive process too much, or, more to the point, without affecting their marathon performance.
