I do quite a bit of running, some lifting and I've never personally experienced this with either. However, I also do a lot of mountain biking and experience this during and after each ride. As best I can describe it, it really becomes of a sense of "flow" and you feel connected to what you are doing while a wave of good feelings hits you. The only similar expression of this that I've been able to come across is how surfers often describe their feelings after catching a good wave.