Here I am at the summit of Wheeler Peak, NM. My round trip from the trailhead to the summit and back must surely count as the roughest single day of hiking I have ever done in my life. The altitude really got me. I was washed out for months afterward.
A couple years later, when one doctor informed me that an old heart attack showed up on my EKG, I immediately thought of that day on Wheeler. The cardiologist I consulted did scads of tests and told me that no, I had not had a heart attack. I told him about my day on Wheeler Peak and asked him what that experience might have been, then. He said I probably ran my body out of calcium.
This is why you're supposed to spend some time acclimating to the higher elevations before putting your body to such a test. I flew out to Colorado Springs, and two days later I was hiking up to Bull of the Woods Meadow. Not smart.Me on top of the worldWheeler Peak, NM 13,167'