"There's a reason we feel pain when we run. The reason is, we will die if we do it too long. "

At some point during a prolonged run the intake of oxygen will be exceeded by the demand of the muscles. At this point instead of using oxygen the muscles will begin to use glucose. The period of time for which the glucose use occurs efficiently is 3 to 4 minutes. After that lactic acid begins to form as a means of storing the energy for use in the presence of higher oxygen levels. In other words, the muscles begin to be unable to work 1 . Olympic and marathon runners are training their cardiovascular systems to greater oxygen intake and distribution as well as some training in the use of glucose by muscle cells. Continued exertion results in the inability to continue the exertion. A mechanism that will occur well before death.

The cramping felt when exceeding the body's capacity is a stimulus to the autonomic nervous system. While we living human experience this as pain, thus also having a conscious reaction in the central nervous system, zombies will still experience the same chemical reactions and simply cease to run. Without the conscious mind to override the autonomic nervous system, zombies may drop out of the race sooner than their living targets. That point would be determined by the ability of the cardiovascular system to take up oxygen and deliver it to the muscles. Asthmatic and smoker zombies will be easy to out run, marathon runner zombies (they must have been caught sleeping?) are going to be real pains.

Rule # 1 : Cardio
1 http://www.scientificamerican.com/article.cfm?id=why-does-lactic-acid-buil

Edit: added conjecture for zombie durability.

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