This is long but I think yiour questions warranted a deeper dive.
Thanks for writing in with these challenging questions. I can see you’ve got our forum mods putting their thinking caps on. It’s good for everyone to have this type of discussion because we can all learn from it. Let me start by echoing Reed’s question of “what makes you think you are aerobically deficient?
Next, let me caution you about trying to translate metrics gathered in a lab study to you in the real world. Your glycogen muscle storage on any give run might be quite different. One thing I have learned over many years is that no 2 athletes respond to the same stimulus in the same way. This is VERY far from an exact science like Physics (which deals with the simplest things at the most basic level) where you can actually write mathematical equations and expect them to represent the real world. Even engineering which deals with more complex real world physics problems has to make assumptions, sometimes very big ones and has large error bands in the answers. That’s the reason for the so called “safety factor” in engineering. It’s because the models at best give you a reasonable guess at an answer.
The body is sooooo much more complex then a bridge or an airplane wing that there is no way to model it mathematically and expect that you will find meaning in that model. You would need extensive testing on yourself; including a muscle biopsy and blood work and a gas exchange test to have any hope of being able to define your starting point.
All this is to say that while it very useful to have science to back up your thinking do not expect exercise science to be like physics. Economists make this mistake the time.
In engineering this is called “False Precision” Because we can measure something to the 3rd decimal place does not mean it is useful to do so.
Now. Back to your questions. My answers may not be very scientific but they come from a lot of observation:
Speeding up at the end of a long run is a very useful training method for advanced athletes. Renato Canova calls these runs “Progressive Distance”. I have used them extensively with high level Cross Country skiers. We never go to the maximum effort like you mention. I think “Giving everything you have” in any workout is a big mistake What I have observed with the skiers is that 3 hours into a long workout (Not fasted), even as the speed increased from Z1 and gradually through the intensity zones up to maybe AnT+ 5bpm for the last 5 minutes (again, never going to max especially in this type of workout is that blood lactate levels remain quite low. Of course this implies a very low glycolytic contribution to the over all energy demand.
My take away is that, as you say, glycogen stores are low, and oxidative metabolism is using fat to provide the energy. Well trained elite athletes can produce high speeds on fat alone. How else can we understand how Kipchoge can run 26 x 4:30/mile? No person can carry enough glycogen to sustain that speed for that long. I have tested skiers who even at maxVO2 effort are still getting 10% of the energy from fat. Most physiology texts will tell you that is impossible.
The reason for the ability to surge at the end of a long effort is due to the glycogen conservation these top athletes utilize. They are not draining the glycogen tank much at all in the early parts of these runs. Thus they have more in reserve for that final kick. If you’ve been draining the glycogen tank so much that it is empty by the end of a race or workout then there is no kick.
I used to regularly test my skiers during a very demanding interval session involving 14km of skiing with 10km of that very hard. I would measure lactate in the rest intervals to ensure it was relatively low. Within 1 min of finishing the last repetition they would do 800m as fast as they could. In the beginning of this type of training blood lactates were higher during the interval part and last 800 was often slower than all the hard efforts. By then end of a 10 week progression they could really kick and the 800 would be very fast and they could produce very high lactates (12-16mMol/L). A great example of glycogen conservation.
I’ve known very highly fat adapted athletes on Keto diets who could still operate at high intensity for a long time. I can only surmise that Gluconeogenesis is providing them with glucose.
In in the end keep your eye on what works and what doesn’t rather than trying to dive too deeply into why. Let the white coats figure that out. We care a bout performance.
I hope this helps.