Lordy, another excellent SportsGeezer post: Tips for hiking at high altitudes.

I did my first hike with notable altitude effects last summer at Lassen Peak, which tops out at over 10,000 feet. Lassen is at best a moderate hike; many Bay Area trails are tougher. On the Lassen Peak trail, everything about the walk — exertion level, perspiration, etc. — felt normal, except for my heart rate, which was racing the way I’d expect after a quarter mile of really steep, difficult terrain. Basic biology: your lungs crave extra oxygen so your heart works faster to provide it.

The thinner air also started giving me a headache and I reasoned, heck, water has a lot of oxygen in it, so drinking more should help that out. And sure enough, a few extra swallows killed the cranial complaints.