Something that actually exists. After strenuous exercise endorphines are released creating a "high" feeling. Its not the same as a marijuana type high. Its more of a lightheaded feeling and you cant feel your body alowwing you to run even further. You wont get it if you run 5 minutes. Most feel the effect after 30-40 minutes of hard running.
Joe: Dude runner's high doesnt exist.
Bob: Thats because you only run a lap then give up you fucking pussy.