Many endurance athletes spend their training weeks oscillating between Z2 and Z3 without ever really committing to one. Easy sessions become a bit too fast, hard sessions lose their freshness, and after a few weeks fatigue sets in. Progress slows down — sometimes it even comes to a complete stop. We’ll show you how to put Z2 back at the center to build your engine, and how to use Z3 with intention, without letting it become your default zone.