If you’re truly carb depleted, you can start gluconeogenesis via amino acids, as well as possibly a conversion of fatty acids into ketones (if duration goes on long enough)
Glycogen replenishment will happen whether you eat a high carb meal after a workout or not. It just happens faster if you do.
If you run 10 miles and don’t purposely refuel, just eat like normal, you’ll have decreased performance if you want to run again later that day.
If your next run isn’t for a day or two, you’ll be back to normal by then just from your regular diet. (People who eat high carb diets habitually have an easier time of this than people who eat low carb.)
My understanding is that if you do not feed your body the carbs it needs to restore your glycogen stores, it will start synthesizing glucose for energy out of your lean muscle and fat. This is inefficient and slow compared with how quickly your body converts carbs. Depleting your glycogen stores builds endurance, but you have to give your body simple carbs afterwards for a full recovery and to see improvements in your exercise capacity.
Also, while you don’t have to eat right after your workout, you’ll feel terrible if you don’t eat at all. I very specifically remember the times I didn’t eat after a long run mostly because I felt sick and tired and dizzy.
Only thing I consume right after a work out is water, sometimes a protein shake. Maybe I’m doing it wrong, but seems to be working for me. I couldn’t imagine trying to ram in a bunch of carbs after a workout. Especially if it was a legs day. I usually feel somewhat nauseated after.
Your body needs enough glucose to keep your body going after your workout. That could come from liver glycogen, it could come from food you eat, or it could come from gluconeogenesis.
Your body would *like to* replenish glycogen stores, but there’s no requirement that it do so right away. The idea that you should eat right afterwards has been a great marketing tool for the supplement companies but isn’t really a requirement.
If you are very low in glycogen stores – let’s say you bonk – then your body is going to do everything it can to create glucose, and includes tearing down muscle tissue and converting it to glucose. Or if you spend long hours – let’s say 4-5+ often – in a glucose-limited state. There’s a reason pro cyclists look so emaciated.
How much this is an issue for normal people depends a lot on how good you are at burning fat and how you train. If you train fasted often, you will conserve glucose and become a better fat burner, and at that point you will preserve more glycogen at the end of a workout. I generally do my long runs (1.5 hours) or medium rides (2-3 hours) fasted and then I might wait a few hours before eating as I’m not really hungry.