Just to add some anecdata as a pile, I run about 1200 miles a year (half pavement, one third trail, the rest track). I regularly get a sort of runner's high after covering 5+ miles at a hard (7 min) pace, or 10+ miles at a moderate (8-9 min) pace. The intensity varies, but it usually sets in after this considerable distance and lasts for 2-4 hours afterwards.