On May 25, 2014, myself, my wife, and a friend hiked Mount Whitney in one day. We woke up at midnight and started on the trail head at about 1:15 AM, and summited around 12:30 PM. We made it back down to the car at about 8:30 PM, just as it was getting dark.
There was a lot of snow on the upper portion of the trail above trail camp. We ended up climbing the chute, which was the toughest two hours of the climb. We kept our crampons on most of the way from trail crest to the summit because of all of the snow. When we got back down to trail camp I didn't feel so hot...I think I didn't eat enough on the way up to the summit and back. So I forced myself to eat and drink a as much as I could and I made it back down to the car.
I think this hike was the hardest thing I've ever done physically. We had trained three out of the four previous weekends on San Jacinto, Mount Baldy, and Mount San Gorgonio. The training hikes definitely helped with the altitude, but there is no substitute for acclimatizing in the days immediately before the "big hike".