There is so much WOW in this post! I've found that it takes ~7km of running for me to get runner's high. Usually it requires a hard effort (up a trail) and then reduced effort (reaching the peak and heading downhill). I've also found that I can help with music. Certain songs trigger it. I won't suggest any because I think it's specific to each person. For me, it's songs I've run to that also have a beat that helps me get there.
This probably is an obvious question but your story makes me ask... have you read Born to Run?
RE: How to tap into flow state when you run