I'm on track for 3000 miles this year. I ran over 2000 miles last year. My shortest run in the last few months is 4 miles, and that's on days when I double with a 6 mile run. I've raced almost a dozen marathons and I run the half-marathon distance a couple times a week. So I think it's fair to say that I "cover considerable mileage." This isn't all just slow miles either. As any runner trying to improve, I also run intervals of varying distances and tempos (where you run about as fast as you could race for an hour, but for about 30-45 minutes).
So I enjoy running. There are runs where I'm relaxed and in the groove and the miles just click by. But that's just an enjoyable experiece, like relaxing at the beach. It's nothing like the high from smoking marijuana. Or the exhileration from something like sky diving that first time. Or post-orgasm bliss.
I've also spoken with a few ultra-marathoners, runners that cover 50-100 miles or run for 24 hours straight. None of us have experienced a "high" from running. Have we all been duped? Maybe we're all expecting the wrong thing?
I'm envious of those mice... :-)