Good job for digging in deep (lay person deep) on the metabolism. I’m not physiologist so I am going to explain it as I understand it and expect that I might not have some of the finer details correct. But, here goes:
Anytime you are in a starvation state you will be breaking down protein for fuel. This catabolic state is why starving people (even those with self imposed eating disorders) loose muscle and look emaciated. This is not a healthy state and not sustainable with severe health consequences. You will also be producing ketones from the breakdown of fats. The liver has the ability to convert ketones to glucose through a process called gluconeogenesis. This is how starving and keto adapted people keep functioning. The brain need glucose for fuel. This is also why high fat adapted people can continue to function at a higher level of activity for longer than those who are not.
– Is having a tiny trickle of food (that can be converted to glucose) intake far superior to having absolutely zero, because it allows you to do a little bit of anaerobic work to “kick start the aerobic system” (which is a term I’ve heard people use in the context of fasted training)?
– Is there a difference between fasted training (probably a little bit of glycogen left) and continuing after bonking (really none left)?
Yes. Continuing till bonking will mean a longer recovery period before glycogen stores are topped up again.
– On a purely metabolic basis, how much is running out of food completely going to slow me down?
In my experience. A minimum of 50%
– If I’m bonking on a long route and I only have one gel to last me several hours, should I ration it, or just eat it now and avoid any potential sticky mess?
Since the brain need glucose save the gel for technically demanding sections or when important decisions need to be made.
Take more gels with you.