WHAT runner's high?!

I have heard tales of a mystical thing. A power so great it can make running actually feel GOOD...The ever elusive Runner's High! In high school I used to ask my cross-country friends how the heck they could run for miles on end all the time...particularly with SMILES on their faces. The response was, of course, "runner's high". Apparently when you run, the pain and torture your body goes through causes your endorphin level to rise. And when it rises high enough you start to feel a wonderful sense of invincibility and euphoria. Now my big question is this--How freaking far do you have to run to start to feel like that?!?!? Because so far all I feel are jello legs and the sensation that I'm breathing through a juice box straw.