When you hit the wall at mile 19 of a marathon (or mile 80 of a 100-miler) it feels like your body is the problem. Your legs are concrete, your stomach is in revolt, and the finish line might as well be on the moon. But what if the wall is mostly in your head?

Emily Balcetis is a social psychologist who studies how vision and perception shape motivation: how what people literally see changes what they believe they can do. She runs the SPAM Lab at NYU and is the author of Clearer, Closer, Better: How Successful People See the World. Her TED Talk on why some people find exercise harder than others has been viewed over a million times.

Zoe and Brendan get into why highly motivated runners literally see the finish line as closer than it is, how Joan Benoit Samuelson used a woman in pink shorts to win the first Olympic women's marathon in 1984, and why the wall is a psychological phenomenon rather than a physiological one. Emily explains why willpower isn't a fixed trait you either have or don't, the if-then framework she calls foreshadowing failure, and how negativity bias means our brains lie to us about our own progress, which is why becoming your own accountant might be more useful than another motivational Post-it on the bathroom mirror. Plus: how Emily decided to learn the drums in a one-bedroom Manhattan apartment, and what it taught her about tracking the kind of progress the brain can't see.

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The featured race is the Haul Ass Ultra Running Festival, Saturday, June 6th, in Erie, Colorado. Eight distances on the menu: a 50-miler, 50K, 10K, 5K, and 3, 6, 9, and 12-hour timed events, which means whatever shape you're in and whoever you're bringing along, there's a way to make it work. Sign up at UltraSignup.com.

The Trailhead is part of the UltraSignup Podcast Network.