Our bodies are complex but very efficient machines.  In a recent article in Healthology it has been described as 3 engines.  The first one providing us with bursts of energy we need for weight training.  This engine runs on protein and does not require oxygen, and therefore is anaerobic. The second one is for half marathons and runs on glucose and also does not require oxygen and therefore is also anaerobic.  The third engine is for running long distances and runs on carbohydrate glycogen and requires oxygen to produce energy; therefore, it is aerobic.