I get that once in a while. Thirty years ago when I swam a mile a morning in my 20s I could almost count on it at about 2/3 of a mile, I'd get a warm rush through my body and then the strains and pains (not many at 27) would evaporate and I felt as if a big wave were pushing me on. I'm sure it's your body releasing endorphins. Sometimes I just want to sit up in the saddle and sing, but that would end badly for all involved.