How Oxygen Affects Energy Levels
Metabolism is also regulated by its patriarch: oxygen. The availability of oxygen determines which metabolic pathway predominates. For example, at the end of the metabolic pathway that breaks down carbohydrates (glycolysis), there is a fork in the road. When there is adequate oxygen to meet the muscle's needs, the final product of glycolysis—pyruvate—is converted into an important metabolic intermediate that enters the Krebs cycle for oxidation. This irreversible conversion of pyruvate inside your muscles' mitochondria is a decisive reaction in metabolism since it commits the carbohydrates broken down through glycolysis to be oxidized by the Krebs cycle.
However, when there is not adequate oxygen to meet muscles' needs, pyruvate is converted into lactate. An associated consequence of this latter fate is the accumulation of metabolites and the development of acidosis, causing your muscles to fatigue and you to slow down.
The more aerobically developed you are, the more you'll steer pyruvate toward the Krebs cycle and away from lactate production at a given pace. That's a good thing, because the amount of energy you get from pyruvate entering the Krebs cycle is 19 times greater than what you get from pyruvate being converted into lactate.
While pyruvate will always be converted into lactate given a fast enough speed, the goal of training is to increase the speed at which that occurs. How do you do this? Focus on increasing your mileage and completing lactate threshold runs.race.