I'm still young (15) but I can think of one thing.
Carrying half my weight on my back while walking (or climbing without hands to be specific) up a hill that had a slope of about two (two feet up for every one foward), sometimes up places where there had been essetially mudslides, sometimes having to take a step as high as your knee (a lot harder with an extra half your weight on your back), which, finally, as I went up about 75 feet up from the lake I got off of, into a swamp were I went knee deep into mud (if you step in the wrong place you could go waist deep).
Call me insane, but that was FUN!
I go on trips to the Quetico side of the Supeior National Forest. You carry everything you need. And by that, I meen that, if you go on two week long trips like me, you were the same set of clothes every day for two weeks. You can take more but you have to carry them.