'Born To Run' is a fantastic book by Christopher McDougall. The book makes a compelling account on how we're born to run and how a lack of [proper] running actually creates injures and accelerates the physical aging process, as opposed to vice versa. Common knowledge is generally that running will injure you and create physical aging. This book was responsible for my re-birth of running and a resulting decrease in some annoying knee pain.
In particular they discuss the Tarahumara and Ultra-Distance events that span well over 50 miles in length and occasionally days in duration. Most people struggle with marathon distances however these athletes have a secret weapon apparently...an ability to tap into body fat stores.
The human body utilizes three fuel sources during long distance low speed activities Carbohydrate, Fat and Protein. Carbohydrate is in limited supply easily running out after a few hours of sustained effort. Carbohydrate is also very important for high intensities and should therefore be rationed by the body. Protein is not favorable for metabolizing as this indicates you're burning your precious muscle as a fuel source.
The main source left being fat which has an abundance of energy that will literally sustain energy at low intensity activity for days if not weeks. Unfortunately, due to high sugar high energy diets and low exercise, our bodies are extremely poorly adapted to mobilize and burn fat as fuel. Resulting in fatter and fatter populations and runners hitting 'the wall' early due to depleted Carbohydrate stores coupled with an inability to burn fat.
In the 'Born To Run' book a character named Phil Maffetone works with many top ultra-endurance runners to help train their bodies to efficiently burn fat as a fuel source. Over time the body must adapt like any other process. The process of adapting to heavy loads lifted will result in larger muscle. Long very low intensity activity will result in an increased ability to release fat from stores and burn it as energy. An inability to do this will seriously hamper any endurance athletes chance of success or event completion.
How then do you determine the correct intensity or speed of your run?
The simplest and most effective way to monitor intensity during cardiovascular exercise is heart rate monitoring. Maffetone recommends calculating your 'fat burning' zone by:
180 - age = Training Zone for Maffetone Fat Burning
The aim is to maintain your heart rate at or below this number whilst walking, running, jogging, swimming, cycling etc. This is the only way to effectively adapt your body to burn fat as a fuel source during exercise. Warning: it can be extremely frustrating attempting to maintain a low heart rate if you are used to training full steam.
The Karvonen Formula is another way for the geek in you to calculate heart rate training zones based on both age and current cardiovascular efficiency. The video below illustrates how to complete the formula: