Runners learn — the hard way, naturally — that there is an invisible barrier at the 20-mile mark known as The Wall, the point where you’ve burned off the body’s ready-to-use chemical energy.
You remember the feeling. Your thoughts, once as free and fluid as your running, turn to viscous sludge, like wet concrete. Your legs abruptly submit their resignation. All vital signs, as measured by your sodden brain, point toward system collapse.
The Wall. Bonk.