Aerobic base building is the foundation you build your future strengths that allow for injury free training. People who train for half or full marathons at one point move into speed training and will experience reaching their anaerobic threshold which occurs when the body can no longer solely fuel workouts via oxygen. The body then begins a gradual change over to a very limited form of energy provision that uses lactate. Anaerobic threshold occurs at 65% of maximal heart rate for novice runners and 85% for experienced athletes. When running becomes breathless and struggling, your pace has exceeded your anaerobic threshold. With progression into the training, you will find that you can run within 6-8% of your anaerobic threshold and still feel comfortable. Regardless of your fitness level, always follow a hard training day with a recovery day.