Age is just a number.

Even if you gorged yourself after your first run, full intramuscular glycogen replenishment takes about 48 hours. It looks like you came pretty close to emptying the tank on the first run. Did you eat during it? The problem with running the double as I see it would be what your body is doing to maintain its blood glucose (and supply to the muscles) during that second session. Without the readily-available energy stores, yes you are burning fat, but you'll also see an increase in glucocorticoid release, which will result in the break down of muscle. Those are just my thoughts. It's nice to see people putting in this kind of effort.

Sam - do you mean you could have all of the muscle glycogen back in 48 hours after running it to near empty?

He is correct on that one as I have even read that it is possible within 24 hrs if you are receiving 50 grams of carbohydrate every hour or so. But you must have all types, simple sugars for the first hour, followed by fructose for liver glycogen replenishment and then more complex carbs after that.

My $0.02: Starting at the beginning...you take off for your long run. You're probably not working at a VO2 max rate that's going to burn through your blood glucose and liver/glycogen stores right off the bat via muscle demand, so it's actually the steady demand from the BRAIN that will be responsible for dropping blood glucose. Glucose is its preferred substrate and free fatty acids have a hard time passing through the blood-brain barrier. (Only in the starved state will you see ketones being taken up by the brain. Think about the psychological side of bonking.)

Anyway, eventually your blood glucose drops a bit and your glycogen stores begin to drop as well to keep blood glucose fairly constant (for the brain). Strangely enough, it's this glycogen depletion that is probably responsible for ramping up lipolysis via increased insulin in the blood stream. As I said before, glucocorticoids are also being released in order to free up amino acids for metabolism.

Your body doesn't like to go too long without glycogen since it basically exists as a metabolic buffer. It's very dynamic in terms of its depletion and replenishment, which is fortunate for us. After your run, your body will keep up the lipolysis as long as you have the fat stores available (ie haven't spent the last few weeks in the desert). The effect will be that FFA's will be converted to glucose and then some of that glucose will be subsequently stored as glycogen in both the liver and the muscle. With food after exercise, you're doing your body a big favor since it skips a lot of metabolic pathways in the process of glycogen restoration. While it's true that carbs are great for this, it's a good idea to eat protein and fat right afterwards as well. Some recent studies have shown that glycogen comes back faster with a CHO and protein mixed intake.

As a rule of thumb, you can expect between 2-5% replenishment per hour. Keep in mind that this is a far cry from being the only limiting factor in running performance.

Interesting on the brain aspect of things. I usually take in protein to carbs at a 1:8 ratio right after. Have you ever experimented with certain amino acids to aid your recovery? I am debating on that one however.. Thanks.

Nope, I never have. My stand by recovery drink has always been Endurox (4:1), which I know has L-glutamine. Who knows what else...