While reading the book from Daniel, Kahneman, “Thinking, Fast and Slow” I came across a great example of how our brain is a huge consumer of glucose.

“Volunteers in one of their studies watched a short silent film of a woman being interviewed and were asked to interpret her body language. While they were performing the task, a series of words crossed the screen in slow succession. The participants were specifically instructed to ignore the words, and if they found their attention drawn away they had to refocus their concentration on the woman’s behavior. This act of self-control was known to cause ego depletion. All the volunteers drank some lemonade before participating in a second task. The lemonade was sweetened with glucose for half of them and with Splenda for the others. Then all participants were given a task in which they needed to overcome an intuitive response to get the correct answer. Intuitive errors are normally much more frequent among ego-depleted people, and the drinkers of Splenda showed the expected depletion effect. On the other hand, the glucose drinkers were not depleted.”

Excerpt From: Daniel, Kahneman. “Thinking, Fast and Slow.” Farrar, Straus and Giroux, 2011-10-24T14:00:00+00:00. iBooks.
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Something to consider is that when you are running under a stressful situation, you’ll burn more glucose and increase your rate of fatigue.  In other words you will hit the wall faster than you expect.  Run faster than your body is adapted for, the higher your state of stress, and the less likely you are to run an overall “faster” race to get that PR your looking for.  Reduce the stress and run easy and it will all come with time!