In this episode of The Trail Running Briefing, we unpack why Zone 3 is one of the most misunderstood training intensities in endurance sport. Often dismissed as “junk miles” or the “grey zone,” Zone 3 is frequently criticised simply because many runners use it by accident rather than with a clear purpose.

This episode explains why that view is too simplistic, especially for trail runners and masters athletes. We explore how well-structured Zone 3 work can help build strong, sustainable endurance, improve climbing-specific fitness, and develop the ability to manage lactate efficiently during harder efforts.

We also look at why combining brief periods of Zone 4 with sustained Zone 3 work can be so effective. Instead of seeing lactate as just a problem, this approach helps runners understand how the body can reuse lactate as a fuel source, while avoiding the excessive mechanical stress that often comes with faster, more aggressive sessions.

The key message is simple:

Zone 3 is not junk when it is used deliberately. The real mistake is drifting into it too often without intent.

This episode gives trail runners a practical framework for using Zone 3 wisely within a balanced training week.

Key references:

• Brooks GA. The Science and Translation of Lactate Shuttle Theory. Cell Metabolism. 2018.
• Faude O, Kindermann W, Meyer T. Lactate Threshold Concepts: How Valid Are They? Sports Medicine. 2009.
• Seiler S. What is Best Practice for Training Intensity and Duration Distribution in Endurance Athletes? International Journal of Sports Physiology and Performance. 2010.
• Billat LV. Interval Training for Performance: A Scientific and Empirical Practice. Sports Medicine. 2001.