Almost every time I am under pack, I get a period of time that I can only describe as hiker's high. My legs feel like they are on autopilot and move without effort no matter what terrain, and my pack feels like it is lifting me up the hills. This weekendo on a 12 mile day hike (with full 20 lb pack for training), it happened about miles 5 through 8. Other trips it may not happen until day 2 or may occur every day...sometimes for hours other times just a few minutes.
I assume it is like Runners High, but am not a runner. Anyone else get this regularly?