Historically, experts believed three factors would predict a runner’s performance in the marathon: VO 2 max , lactate threshold , and running economy . However, in recent years, a fourth factor has emerged to explain why some runners can look nearly identical across the “big three,” but perform differently on race day—that is, physiological resilience or what some call “durability.” This metric is essentially a measure of the difference between your potential and what you’re actually capable of in a long-distance race. It’s tricky to measure because doing so accurately would technically require a visit to the lab and multiple VO2 max tests (one in a fresh state; then another after several hours of running).…