How can I preserve my hard earned muscle mass on endurance days? How do you eat and refuel on those days?

I like going for multi day hikes, bike tours and raves. I do not know how to fuel my body properly. I am at the end of a cut and I am afraid I will lose muscles on those days. Hence, I'd like to be prepared for the future.

My naive approach is to roughly calculate the kcal it takes and eat that more on that day. E.g.

C = kph * hours

where C is the total calorie consumption and kph stands for kalories per hour.

I think this is too naive. I read that people eat gels during marathons. Is that also the way to go for this? Is there something comparable that is natural? I am not sure which source is good and trustworthy and which source is just pseudo-science.

E.g. this post from muscleandfitness which states

additional calories burned are replaced using a macronutrient ratio of 75% carbohydrates, 20% protein, and 5% fat

Or racecast writes

• Start fueling at 40–45 min — not when you feel tired.
• Target 60–90 g/hr of carbs for marathons (30 g/hr if you're new to fueling).
• 90 g/hr requires dual-transport fuels (glucose + fructose) + 4–6 weeks of gut training.
• Chase standard gels with water, never sports drink.
• Hydrate to thirst — 400–800 ml/hr is typical; heat raises this.
• Caffeine gel at 45–65% of race time for peak effect at miles 18–22.