The stretch from Lone Pine Lake to the Trailhead seemed absolutely endless. We were very near total exhaustion when we got to the John Muir Wilderness sign and knew there was only about a half-mile to go. Mt Whitney had exacted a toll - but we bagged the summit in one day, my 6 th state high point. I don't recommend others try it in one day, but I'm proud to say I did. Then back to Ontario and our planes home.
Weather can change quickly and it definitely gets colder up high.
Also rain gear even if the sky is clear, because you will likely have to travel through some snow. Likewise take an ice axe if you have one. Even 10-15 pounds gets mighty heavy when you're trying to move fast, uphill, through snow and ice. Also, the trail is very clean and everyone needs to cooperate to keep it so. Pack out your trash and use the solar toilets at Outpost Camp and Trail Camp.