I've done some of the New Hampshire Progressive Rail Trail system. I, along with another cacher, did 52 caches in one day, from about 6:30 AM to about 5:00 PM. I stopped because I got my shoes wet early in the day and my feet were getting VERY sore. I ended up with blisters on the bottom of my feet for a couple of days.
If you do this trail, take along an extra pair of dry shoes. It had been raining the day before we did the trail and some of the tracks were submerged (no deeper than my shin, but there was no way around it). If I had some dry shoes, I probably could've done some more caches before the daylight gave out.
Oscilating between remarkable brillance and sheer stupidity with amazing regularity.