You must not think that it's just a long day hike.
It's all about the altitude. Unless you've done a lot a 10K+ altitude hikes, you DON'T know what you're getting yourself into.
Some people won't even notice it.
Others die from it. Yes! People have died on Mt Whitney just from the altitude.
Read posts on both boards
Read the FAQs on whitneyzone
Hang out on the boards and follow what's going on between now and your trip
Keep a close eye on what's going on right before your trip, in particular the weather
Plan on starting the hike EARLY so that you get to the summit by noon or 1pm at the latest. The reason is thunderstorms tend to form after that.
Just like with Half Dome, on Mt Whitney you don't want to be near the summit if there's any clouds nearby that might result in lightning
Head back down if the weather turns
Pick a turn-back time. No matter where you are, not matter what you're doing, head back at that time!
Don't get get-to-the-top-of-the-peak fever. Getting to the top isn't important, getting back down IS! Don't hesitate to turn back.
Play attention to hydration and electrolyte balance
Consider bringing rain gear, even if just a paper thin poncho
Consider what you'll need to keep warm. It gets a lot colder as you go up, and if it's windy ( it's almost always windy up there ) you'll need it even more
Bring a head lamp, even if you think you'll be back before dark
Hope for the best, plan for the worst
Leave No Trace! Use the Wag Bag if you have to, and pack it and the TP out
If you find any trash that some deadbeats have left up there, consider packing some/all of it out.
Take lots of pictures!
I know that day hiking Mt Whitney is more than I could ever do.
I backpack it, and make it a 4+ day trip, one day to Outpost Camp, one short day to Trail Camp ( Consultation Lake area is nice ), one or more days to do things, one day out.
It's what I can handle, and gives time for acclimation to the altitude