Obviously, when we run in a marathon we are stressing, straining, and depleting our bodies--to an extraordinary degree. I have found that in order to keep going beyond say fifteen miles or so, I have to take some pretty strategic steps right from the start of the race.
First, there is the matter of staying hydrated. The problem is that when we sweat, some of the water content we lose through our pores is actually taken from the blood supply. When blood volume decreases, heart rate needs to increase to pump the same amount of oxygen. Therefore, dehydration is always accompanied by a decrease in pace at a given heart rate. Studies of endurance athletes have shown that there is a 3% decrease in pace for each 1% decrease in bodyweight due to dehydration. So for example, a 190-pound runner like me could lose as much as 2-3 pounds of sweat per hour when running on a warm day. Thus during a marathon, I could conceivably lose 8-12 pounds resulting in about a 15% pace reduction. That could be as much as an extra minute or a minute and a half per mile. The only solution to this dilemma is to stay hydrated during the race. And the thing is, we can't rely on our thirst because by the time we are actually thirsty we've already started to dehydrate. It is equally important to not drink too much. The standard rule of thumb is that during a marathon we ought to drink small amounts often--so, for example, I never skip a water station on the course. But, I also try never to exceed 4-5 ounces (just a couple of good swallows) in any 10-20 minute period.
Second, there is the matter of keeping our body systems in balance. The problem here is that when we sweat we lose a great deal of our stores of sodium and potassium--which are the minerals our bodies use to balance their fluid and electrolyte levels. Thus, replacing lost sweat with water alone can be nearly as dangerous as dehydration. Hyponatremia, or sodium depletion, can lead to many of the same symptoms as heat stroke--headache, nausea, vomiting, extreme fatigue, and dizziness. A lack of potassium can lead to paralyzing muscle cramps. Thus, it is important for us to supplement our sodium and potassium stores with sports drinks. I try to get a couple of swallows of Gatoraide or something similar at about every other water stop after mile five or so.
Finally, there is the problem of staying fully fueled for the three to five hours that we're actually out there on the course. Our bodies are constantly burning carbohydrates. As the miles add up, our carbohydrate stores become progressively depleted and our bodies will try to conserve what's left by burning fat. The problem is that fat is not a particularly efficient energy source. So, when we start running low on available carbohydrates, we inevitably slow down and wear out. That's where energy bars and gels come in. I usually try to start introducing small amounts of energy supplements around mile eight--and then every four or five miles after that.
It is a lot to think about. But then, during a marathon, there is plenty of time to think.