Muscles will eventually run out of glycogen, though, then the liver produces glucose from muscle tissue itself. The body's pool of protein is actually mostly just muscle tissue and the little bit of waste products inside cells. We don't store protein anywhere else, and the blood has a very limited capacity to carry protein. Fasting occasionally helps clear out the waste products of metabolism, but too much fasting can also be detrimental to lean muscle mass.
Carbohydrates can actually be "protein sparing", and if they are consumed in small amounts combined with exercise, they won't be stored as fat. They allow your body to better use protein to build muscle or burn fat, instead of burning protein as fuel.
The volume of workouts you do even if you double, triple it won't come close to getting your glycogen levels dangerously low.
If you're worried about losing muscle mass, it's better to load up on easily digestible protein food with minimal amount of carbs before the workout if you wish to lose body fat as well. A good example of this is unsweetened cocoa powder like that sold by Ricoa or Hersheys. You can turn it into a beverage but if you're trying to lose fat, don't add sweetener unless you're using calorie-free sweetener. It will taste bitter without sweetener. But don't put too much, just enough to remove some of the bitterness. Even calorie-free sweeteners can mess up your hunger hormones so be careful. Best if you don't put any sweetener at all. Dark chocolate is NOT a good substitute because it is still loaded with sugar and other potentially toxic synthetic ingredients.
You have to trust me, I didn't come down to <7% body fat just reading articles and exercising. I did the work, did experiments on both exercise and nutrition, prayed and received guidance.
You're on a very different training regimen with different goals. I'm just aiming for the equivalent of getting 7,000-10,000 steps a day, and trying to preserve muscle mass as much as possible.
I think it's a case of moderation being best when it comes to cardio. If you enjoy six hour bike rides, then that's reason enough for you to engage in them, but I don't think there's good evidence that is necessarily healthier than just getting some walking in most days of the week.
I never recommended you to do such volume. At most, I'm only recommending you 1/3 the length of the cardio workout. 2 hrs in just one day each week and then much shorter exercise session in other days of the week like 30 minutes each day.
A few weeks ago, I did only one 2.75 hrs of cycling ride per week and 20 minutes of lifting each day and couple of months before that and did pretty well with it. I only gained 2 lbs of weight (mostly in fat) and not any more and maintained it for several weeks.
I'm in total agreement with you.
And too much cardio volume definitely has its downsides: some professional cyclists, after all, develop some disturbing cardiovascular abnormalities (enlarged hearts, atrial fibrilation and other arrythmias), not to mention eating huge amounts of food for no other reason than to become a human dynamo, but for them, it's all part of the risks and costs of playing their game.
My weekly volume still pales in comparison to the pros. Pros do at least 14 hrs of weekly training. Many upwards of 20 hrs /week and some even pushing it to 31 hrs/week. I only do 8 hrs/week at most.
I don't have the problems you listed.
I don't even eat a lot I only eat 1,600 Calories a day including the day I did 6 hr bike ride.
I've also done these rides fasted and sometimes without drinking anything before and during the exercise. No sane article will tell you do that because the results can be fatal. I won't recommend anyone to do it. But when I workout, I only drink when thirsty and I have indeed gone 6 hrs continuous bike ride without feeling thirsty even in the summer heat so I didn't drink. I came out perfectly fine, no issues at all.
God did promised these things in the Bible. You will no longer thirst, you will no longer get hungry, you will run and not grow weary, the sun won't beat down on you anymore. Maybe I'm taking things out of context but seems to be working out for me. I still won't recommend going these extremes. Definitely don't do it, stay safe!