I am a runner, so I kind of always train, but I do more hiking and stairs when I'm planning a hike up Mt. Whitney. I would say that, generally, if you are in shape to run a marathon & easily hike a 10k+ peak at a reasonable clip, you can hike Whitney in a day. It's a bit more civilized to do this hike with an overnight or two on the way, however. So beautiful...more time to enjoy the trail!