I've never been a runner, but I used to be on the swim team in high school and would get runner's high when we had to swim laps; the first 5 or so laps would be hell, but after that my body would just sort of take over like a robot and I would feel like I could swim for hours.
I used to look forward to it ("five laps of hell, and then paradise").
I don't think it's really the same as the feeling I get during chi kung, but my opinion may change (interesting things are starting to happen, but the jury is still out).