The Magic Zone
Why such a focus on training in Zone 2? The body’s two most important energy systems are aerobic (requires oxygen) and anaerobic (in the absence of oxygen). In Zone 2, you train using only your aerobic system, so your body is forced to learn how to do that better. This builds a strong base of energy that is always available, at any speed. When you train in Zone 3, which is where most people spend their time, your body uses energy from both aerobic and anaerobic processes and neither system gets stressed enough to meaningfully improve.
Physiologically speaking, training in Zone 2 improves your ability to transport fuel and oxygen to your muscles. You also get better at using your body’s fat for fuel, which in turn allows you to conserve glycogen stored in your muscles. This improves your ability to race or perform over longer distances. Once you have built a strong aerobic base, you’re ready to benefit from the higher intensity workouts you will do as you progress.
Most runners—especially recreational runners—never run easy enough to stay in Zone 2, thus missing the opportunity to build the strong aerobic base that allows you to run faster for longer. Plus, if you train too hard all the time, you never allow your body to properly recover from training days, which can lead to fitness plateaus, injuries, and sub‐par performances on race day.