Threshold running is, as you probably know, the single best way to improve your aerobic fitness. Basically, you get "the best bang for your buck" during a threshold run, as you need to run at your threshold for only a few miles to gain fitness. I make sure that the athletes I work with are running at least one workout per week that is some version of a threshold workout.

As important as threshold runs are, I encourage you not to become addicted to wearing a heart rate monitor during one. Why? Because your heart rate can vary from day to day, depending on fatigue, temperature, and even hydration level. I was reminded of this when two athletes I work with ran a threshold run together and the two runners had vastly different heart rate values at the end of the workout. However, as I observed them throughout the run (as I was on a bike), they were both running controlled and at a true threshold effort. Their perception of the effort was much more important than the heart rate value. So make sure you do your threshold workouts, but don't obsess over your heart rate; rather, use those values as a guide.