I’ve always been a data nerd when it comes to running. I love learning more about my body and how it runs. As a result, I’ve tried all kinds of weird running tech: stick-on sweat patches and continuous glucose monitors that sync to my phone; smart insoles that track my stride; and earbuds with built-in A.I. coaching. And, of course, like most runners, you won’t catch me without a running watch (sometimes two!) strapped on my wrist. During races, I’ve focused on just about every pacing tool my watch has to offer—average pace, lap pace, projected finish time, even real-time “race predictor” timing. But starting last year, at my coach’s suggestion, I simplified everything down to a single metric: elapsed lap time. Elapsed lap time simply shows how much time has gone by during a specific workout segment—during a race, that means each mile.…