"Preparing your body physically is vital to your success in overcoming challenges and reaching goals. You must be in as peak a physical condition as possible to withstand both the physical and mental rigours you will encounter.
"When you start a serious and consistent training programme, your body begins to transform itself in various ways. Your bones will start to thicken in your feet and legs and become more robust; your circulation will improve; the walls of the heart thicken and its chambers increase in size; your joints are lubricated and work more efficiently; the muscles in the diaphragm become stronger and more efficient, which aids breathing; the number of blood vessels that supply each muscle cell is increased and the mitochondria multiply, enhancing the cell's use of oxygen to produce energy. The rate at which the cell burns fat to produce energy is also increased. Overall, the fast-twitch muscles in your body increase their resistance to fatigue. The respiratory system is cleared of mucus while running, which enables you to breathe more clearly. The skin becomes more efficient in regulating heat and resisting cold, and more efficient perspiration tends to have a cleansing effect that reduces the likelihood of catching skin-related infections." - The Elite Forces Manual of Mental & Physical Endurance, Alexander Stillwell, (c) 2005, p. 25