The gold standard of long distance running and marathon training is to build your aerobic capacity by increasing cardiovascular endurance. This is done by systematically increasing your longest run, and thus your total weekly mileage, at a moderate pace with an intensity of approximately 70 to 80% of maximum heart rate.

The energy system that is used for long distance running at a moderate pace is the Aerobic Oxidative System, which utilizes fats, carbohydrates and some proteins. By definition, aerobic running means to run "with oxygen". This system requires that the heart be able to pump large quantities of blood to the working muscles that are then able to extract the oxygen from the blood to increase your endurance.

However, incorporating interval running into your training program will enable the Anaerobic Lactic Acid System to be used. By definition, anaerobic running means to run "without oxygen". This system allows intermittent work to overload the heart within a shorter period of time by alternating faster running with rest periods. By doing this, more cardiovascular work can be accomplished in the training session leading to both enhanced aerobic and anaerobic capacities.

Other improvements include:
1) More effective utilization of fats and carbohydrates.
2) The increased buffering capacity of muscles (their ability to neutralize acids) thus delaying the onset of fatigue due to lactic acid accumulation.
3) Increased "stride frequency" (the amount of times your leg turns over, counted in one minute.)
4) Improved "stride length" and better running form because faster running brings the leg higher off the ground.
5) Eventually, increased speed during your long distance runs…
6) …which of course will eventually lead to faster race times!!!