Cardio Respiratory Endurance falls into two categories.

Aerobic Endurance

The System

Anaerobic Endurance

This is where the body is working so hard that it needs more oxygen than is being supplied and goes what is known as "oxygen debt".

This is where your body is working at a level where the demands for oxygen and fuel can be met by the body's intake of oxygen.

The Key to building your cardio respiratory endurance is to increase the speed and/or distance you can run without going into "oxygen debt".