I have to generally echo this sentiment. I'm at about 1,900 miles for the year and will end up with about 2,500 by New Year's (50 miles per week avg for the year). Most of the time my runs are enjoyable, except maybe the day after a hard workout. Sometimes I'll hit a stride and end up going much faster than intended and the miles click by and I'm amazed at my pace and what I've done by the end.

The closest I come to "runners high" is usually on those days where it's been a bit hot for long periods and then a cool front blows in. I'm practically giddy for the whole run and feel amazing. And I run races from 5k to 100 miles. I really, really enjoy racing, but I think it's the competitive aspect of it and not from some runners high.