Many training zone systems, especially those automatically generated by the Garmins and Suuntos of the world are based on estimates of your anaerobic threshold as estimated by those devices.
A much better way to go is to self-test both your aerobic and anaerobic threshold so that your training zones are based on actual measurements that accurately reflect your current fitness level. This video teaches you how to use the aerobic and anaerobic threshold tests to anchor a simple four-zone training system.
For our full guide to conducting self-assessments of aerobic and anaerobic threshold tests read this article.