The aerobic threshold allows you to get fitter at running for longer.

The anaerobic threshold is shorter and more intense as in interval running. This is the point at which lactate is produced. If you run at this level, you can improve your body’s tolerance and your ability to exercise while generating lactate.

Over time, your body becomes better at using limited amounts of oxygen so that your heart beats more slowly and has to do less work for the same results. This means that you can keep going for longer.