I've run a few marathons, including a couple of Wild, Wild West Marathons in Lone Pine and a marathon in Death Valley.
Day hiking Mt Whitney is harder in my opinion. On top of that, the training is harder to get because you have to drive to a place to get elevation. Downhill is tougher on the joints. Marathon running is usually pretty flat (though I only ran off road marathons) and you can just leave your front door to get started.
I think an overnight is easier than a marathon, with the exception of the acclimatization. Day hiking is hard and you really can't stop anywhere. When you are at the summit, you have to go back and by then you are pretty tired.
On election day, my vote:
Mt Whitney day hike.....1
Marathon................0
Now.....Badwater......that's another story!

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