The chapter introduces the traditional understanding of endurance training that "if a hard day doesn't hurt, it hasn't worked," and argues that the model reverses this: the fastest runners accelerate not by relying on exhaustion, but by staying below their limits. It then outlines the book's roadmap: history, physiology, the logic of the system, and its application in the field. It describes the method's origins in the Scandinavian high-volume tradition, highlighting how its originator, Marius Bakken, developed the double-threshold day through over five thousand lactate measurements of his own blood, and how the method spread worldwide through the Ingebrigtsen brothers. It emphasizes that the model didn't begin with the Ingebrigtsens, explaining that lactate is not a poison, but a fuel and signal used. It introduces the blood-lactate curve, the two milestones, and the balance between production and cleansing. It demonstrates that speed at the threshold predicts performance better than VO₂max and that almost all distance races are aerobic. It explains the fundamental trade-off: keeping each session below LT2 reduces costs, thus allowing for more frequent training and a significant increase in quality volume. The text explains that the threshold is an intensity band, not a tempo band, and why sessions are intervals rather than continuous running; it places the model within the polarized/pyramidal debate. It describes the signature unit "double threshold day," weekly rhythm, base-period logic, blocks, and altitude training. It differentiates the model from the Canova "custom block," which appears similar on the surface. It explains the blood-lactate test, methods for finding the threshold on the curve, proxies used when a lab is unavailable, and the readiness system; it exemplifies how zones are extracted from real test data. It shows that the target band is not arbitrary, but defined by two thresholds. It emphasizes that elites often remain more cautious and that these numbers are approximate, not universal. It explains that the signature of success is a downward shift of the entire lactate curve to the right, the cellular adaptations behind this, and why a higher threshold leads to race victory. It compares middle and long-distance emphasis, presents representative weekly plans, details adaptation for developing athletes, and elaborates on the five-zone framework.