The classification of products into the stress ranges provides a guideline for differentiation. However, the energy supply is ultimately decisive for the use of a product depending on the duration and intensity. The glycogen stores are the limiting factors.
With a total glycogen reserve of around 2,600 kcal, 150g / 600 kcal are in the liver and around 500g / 2,000 kcal are in the muscles. With an hourly consumption of between 600 kcal and 800 kcal in the area of zone 1 to zone 2, the liver glycogen is roughly used up after one hour. The muscle glycogen stores are still about half full even after 2 hours with a supply of 30g of carbohydrates per hour.
However, in the first step, the glycogen stores from the liver (150g, 600 kcal) are activated. To activate the glycogen stores in the muscles (approx. 500g, 2,000 kcal), the muscles need more intensive exercise. This means that these glycogen stores are not activated as strongly in zone 2 (fat burning training). This means that more than 2 hours of fat burning training with 30g of carbohydrates per hour also leads to an increased feeling of exertion.
This is also the reason why a fasting run for longer than 1 hour makes no sense. Then the liver's glycogen stores are used up and the muscle glycogen would have to be activated.
Fat burning always occurs in combination with glycogen burning. Fat burning requires atoms from glycogen burning to work efficiently.
Now you want to do basic training for over 2 hours. To do this, however, you want to avoid the feeling of strain as much as possible and be well supplied with energy. The energy supply plays a major role here and from the start you should therefore also provide the appropriate amount of exercise. This protects the liver glycogen stores (and also the muscle glycogen stores). In addition, the body gets used to the energy supply from the outside with exercise (glucose/fructose). For this, I used a special composition of the various glucose sources during exercise (we are the only ones who do this) and combined it with 1:1 fructose. The fructose goes directly to the liver as a replacement for liver glycogen.
However, if you are exercising in a controlled, relaxed manner and there are no peaks in the load, it would also be possible to work with a base of 60g (double portion). This is best done on an exercise bike/smart trainer, where the load can be controlled extremely well. This would result in particularly good fat metabolism training. It would definitely make sense.