JP: Fitness is truly as important as acclimatization. If you can do a Clouds Rest hike from the Yosemite Valley floor in a day, THAT will prepare you for a Whitney Hike. It is only about 3 miles shorter than the Whitney Trail, and gives you the nearly equivalent 6k' elevation gain. Whitney is definitely a step up, since the summit is 4500' higher than Clouds Rest -- where the altitude sickness can become an issue.
