I ran everything from the 800 to cross-country competitively in college, and I've done a few marathons since. I firmly believe there's no real runner's high, at least not how people think. There is definitely the feeling of being really messed up, but there's not much euphoria that goes with it (unless you win a race or something).
The most messed up you'd get would probably be after running an 800, since this is the race that keeps you in the absolute worst state for the longest - it's basically a long sprint, so your body is being pushed to the limits on the anaerobic system. Yes, most decent runners probably dip below their anaerobic threshold for 10-20 seconds while sprinting at the end of a race, whether it's a mile or a marathon. And I'd be willing to bet most people running casually or doing a marathon never actually hit that threshold. But when the best people run the 800 they're in that zone for more like 70-80 seconds. (Other runners in, say, the mile or 5k might approach that level as well, but that would be exceptional tactics.) So the strongest running high is basically the feeling of sprinting your hardest for over a minute ... without breathing. Doesn't feel very euphoric.
The "pain" part of running, if it's not the mechanics of a body part, is caused by the buildup of lactic acid in your system. Once it builds up, you can't replace the oxygen you need and you'll inevitably slow down, usually in a drastic way like when people "hit the wall".
The marathon runner's high is something different. That's what happens when you dip under your aerobic threshold and stay there for a decent period of time, up to a few minutes at a time. So in order to get the same oxygen-deprived feeling, you've got to dip under that threshold at some point in the race. Most people only do this within a few hundred yards of the finish line, but many runners will experience it during a surge in the race. Basically, the aerobic threshold mimics what happens with the anaerobic one, and you'll get more and more oxygen-deprived and messed up until you can ease up on the effort. In both cases, there's so much lactic acid buildup that, for me anyway, the euphoria can be displaced. Many people, though, experience oxygen debt differently.
If you're still reading (!) there's a third version of "Runner's High" that gets circulated: Basically, it's the feeling of being "in the zone" while your aerobic system takes a little of the edge off your brain by raising the lactic acid levels and thus depriving your brain of some oxygen, but you stay at a comfortable level. (It can also just be an effect of something like a good mood while running.) That's also the level where you'll crank out endorphins to deal with the imbalance, and you'll be able to experience the good side effects of those without the pain of taking your body to the brink of either the sprinting or endurance threshold - the place where you get "runner fucked up".