My highest mile day was a week and a half ago. Did 35 miles as part of a training regimen to get myself in shape to do an ultra/speed hike in a couple of months.
I learned a lot about regulating body temperature, the amount and type of calories I need, and how much water I need to treat, carry, and drink. A lot of these were details I never picked up on smaller hikes, because I never hiked 12 hours without a legit break before.
I have to say, I felt pretty ill toward the end of the hike. This was because I realized the morning of that I had less food than I thought I did and needed to hit the trail early, so didn't have time to go shopping. I ended up bringing only 1500 calories, and it was stupid. I should have waited until the store opened, grabbed something, and then bit the bullet and hiked a couple of hours in the dark instead. Lesson learned.
The good news is that I discovered mental fortitude that I didn't know I had. There was a section at mile 32 that gained 600 feet of elevation in a half mile. Not so bad under normal circumstances, but after 32 miles and on a very empty stomach...
It was super hard, but I was proud of myself for pushing through it.
Also saw some cool wildlife on this trip, that I obviously would not have seen on a shorter one. I nearly stepped on two separate black racers about 12 miles apart, one was around four feet long. Saw a HUGE wild turkey -- biggest one I've ever seen. Spotted a Pileated woodpecker, which is a bit of a treat around here.