Ari and I stopped in our tracks, trying to catch our breath as we racked our brains thinking of what to do next. We had broken the cardinal rule: we got separated from the group. Just a few minutes before I had been thinking the autumn leaves were beautiful. Now the shades of green, yellow, and red reminded me of infected skin and dripping blood. Ahead of us, the narrow path was clogged by a half-dozen staggering zombies.
We explain the science behind the runner's high and how you can get one during your next workout.