Maybe you're simplifying for the sake of brevity, but this notion of "use up all your blood sugar" and "switching" (as if binary) between glucose and ketones is

Maybe you're simplifying for the sake of brevity, but this notion of "use up all your blood sugar" and "switching" (as if binary) between glucose and ketones is pervasive and frustrating. That is not how things work. Unless you have some metabolic disorder, after your hour+ run your blood glucose is still going to be within, maybe on the low end of, the normal range. Even if you've completely depleted your liver of glycogen, which is unlikely for anything short of marathon distance unless you were already low. If your blood sugar was low (never mind gone) you'd be in serious trouble, dead before long if not rectified.