Most importantly, when the name of the game is deterrence, you always want the enemy to be unsure what action will provoke which reaction, especially if the reaction choices include devastating responses.
-- An anonymous student of General LeMay
The second, here, is entitled "Building Your Distance Capability," which nicely dovetails with these observations from Dick Winter's Beyond Band of Brothers:
Physical fitness is another prerequisite for success. I freely admit that I was blessed with a sound physical constitution, but whenever possible, I took the opportunity to improve my physical stamina. Because I was in such good physical shape, I easily survived Toccoa. While men washed out on a daily basis, the contingent from Easy Company that completed the training and earned their wings at Fort Benning were tough as nails. Not surprisingly, I felt that I was in the best physical shape of my life as Easy Company prepared for the invasion at Aldbourne. This did not happen by accident. Following a rigorous day of training, I would take a run every evening following tea with the Barneses. As they were on their way to bed, I would say, "Well, I'm going to take a walk." I would go out and run for several miles even though blackout conditions were in effect. Then I'd come home and go to bed. Because I was in such good shape, my fatigue level never reached the point of physical exhaustion that sometimes contributes to mental exhaustion and, ultimately, to combat fatigue. We all experienced sleep deprivation at times -- that is the nature of stress -- but a physically exhausted leader routinely makes poor decisions in time of crisis. (pp. 286-287)
Winters sums this up in his third principle for success in a list entitled, "Leadership at the Point of the Bayonet.'
Stay in top physical shape -- physical stamina is the root of mental toughness.