Heart Rate Training, by Mark Allen.
I’ve had a lot of success with heart rate training, primarily for long-distance races. However, there is considerable debate over how much training is actually required to achieve good results. Some say you can’t really get the benefits with only 15-20 miles of running. Others say the distinction between fat and carb-burning zones is incorrect, and that energy systems are more of a spectrum rather than divided into zones. The question for most athletes is, how do I achieve maximum results in minimum training time without getting injured?