I occasionally get something resembling a "high" when cycling. I wouldn't necessarily attribute it solely to a hard effort, or at least in my case. Mine seems to be a combination of moderate effort along with the right scenery. When I'm riding my winter routes, down lower in elevation, I don't get nearly the same sense of euphoria. However, with the "dramatic" scenery on most of my summer routes, I do get the "high." Oddly enough, at higher effort levels, the feeling is very much diminished.
I also do get what I consider "being in the zone." It's a different feeling for sure and something that usually happens in higher-stress environments like when I'm mtb racing, descending a technical descent, or spirited riding on one of my motorcycles. My thought processes don't become automatic like some people describe. Rather, time seems to slow wayyyy down and I feel like I have the ability to analyze every turn, or section of road, trail, etc. To me, it's like extreme mindfulness...no thought processes are being wasted on stuff that doesn't pertain to the current situation.