Our bodies are not typical engines

We (in the running community) sometimes liken our bodies to a car talking about fuel efficiency and maximum power or speed. Well that's not entirely correct. Our body has several different metabolic processes that can occur simultaneously. Specifically aerobic and anaerobic respiration. Additionally, and I'm hazy on this because I haven't taken any biochem in awhile, our bodies convert fat to glucose or something like that so that when we are working aerobically we are burning both strait up glucose and glucose from fat. Take that a step farther so that we need a little bit of anaerobic respiration to do whatever we are doing. So now we are burning everything we have fat, glucose, and more glucose. Now the body's supply of glucose (from glycogen) is limited. This is why in marathoning the "wall" is so famous. The distance of a marathon is short enough that people can try to run hard the whole way and it is long enough that you can't run hard the whole way. So many runners get into the 20+ mile range and hit the wall because they ran out of sugar and now their body is forced to burn the glucose from fat which takes a little longer to metabolize. The problem is that while you can "run" on fat as long as you can stay awake it does not provide as much energy "quickly" as sugar (glucose). This applies to just about every exercise not just running. Although, lower intensities (walking), activities with frequent rests (sports involving whistles), or eating all contribute to making it possible to do those for long periods of time. Our bodies are more like a tribrid that will change metabolism depending on the exertion.

A few other comments on metabolisms:

• Before breakfast you have less sugar in your system so you burn up to 3x as much fat than later in the day after you start eating.
• I have about 11 pounds of fat and thus 38,500 calories of fat fuel in me, in theory enough to run well over 300 miles continuously, now, if I wanted.
• I have hit the wall in the 400, 800, 5000, 10,000, and a number of 15+ mile long runs, notably an 18 miler in Colorado on Gold Hill. So our bodies are complicated.