Hiked up Whitney a long time ago with my then-girlfriend. She got altitude sickness about 2/3 way up, even though we had been careful staying overnight at the halfway point to acclimatize. She couldn't wake up and get out of the tent. Wouldn't eat or drink. I was worried about embolisms. We had to turn and head back down. It was pretty scary.

Read Krakauer's Into Thin Air. You don't want to mess around with altitude.