In order to not have anything left after you cross the finish line, it would be ideal to make it there in the first place. I mean, one way to "leave it all out on the course" is to run yourself into the ground early on... but SK said that the elites ran a fairly consistent pace throughout, not they went out hard and died (the ones who win at least). Inherent to this is the ability to learn how best to use the energy reserves that your body has available.
The purpose of long runs is to teach your body to burn fat versus glycogen during endurance events. Basically, fat results is a more stable, longer-lasting energy source. By relying on it, you reduce the amount of carbohydrates you use, and save your glycogen stores to the end of the race when you just need to power through the last mile. Coach said that relying on glycogen stores alone will get you an hour. Obviously, taking in fuel like GU and Gatorade will stretch this out a bit, but you can't replace what you're losing fast enough to get you to the end of 26.2-miles. When you run low on carbohydrates, you begin to feel fatigued, and eventually hit the wall.
From a physiological perspective, the body preferentially burns carbohydrates during high effort levels (aka fast races / interval workouts). However, as it is more difficult to break down fat, the body will also burn carbohydrates during longer efforts if it gets stressed. If you always run your long workouts at a fast pace, your body remembers this stressed state, defaulting to it and burning through your glycogen stores. Conversely, if you teach your body to rely on fat stores, it will preferentially burn these. And, being that you can only break down fat in the presence oxygen, this means staying aerobic throughout the run.
During base training, consistency is the key to helping this process along by increasing mitochondrial production (mitochondria are responsible for generating energy). Greater gains are achieved with higher mileage runs (i.e., one 90-min run is better than two 45-min runs), but diminishing returns are seen in terms of the time needed to recover once you hit 3+ hours. For more information, this
article is pretty comprehensive (I'll probably come back to some of the later sections in another post, but this is one is already getting pretty long).
I have always been kind of nervous about running my long runs "too slow". I mean, this isn't the first time coach has commented on our inability to run easy. I thought about what he said throughout the run (which we actually did a good job of running at a reasonable pace... I will never call Turkey Creek easy). Finally, with about a mile to go, a lightbulb went off in my head (aka the Espresso Love GU kicked in). I've never been able to accept the fact that I should run slower than my marathon pace for a long run because, if I can't do it for shorter distances, how am I going to do it for 26.2? Then I realized that when I train for 5K's, 10K's, half marathons, really anything else but a marathon, I don't feel the need to run at those paces for 3, 6, or 13 miles (or really at any time other than track workouts and tempo runs). In fact, I spend the majority of my time running much slower than that, trusting that my training will get me to where I need to be on race day. Why, then, do I treat marathons differently? I couldn't come up with a good answer to this question (and if I do I'll let y'all know). And, being that (1) coach knows more than me, (2) the science backs coach up, and (3) I didn't feel like crap after the long run that day... I think I'll try keeping my long runs easy and aerobic until I can break 1hr for a marathon :-).
