I don't know if you've ever experienced playing mental games with the distance to assure yourself that you are getting closer to the finish. "In two miles I will be 1/4th of the way to 2/3rds mark..." If I start doing this stuff then usually something is wrong.
Indeed on longer distances emotions become so strong. I did an Ironman a few years ago (and I'm overdue for another one) and it was a very intense day; despair and sadness to intense bliss.
I've still got a 50 miler run and maybe a 100 miler on my 47 Things list yet to check off. I haven't pushed it nearly far enough yet.
Sometime look at the website for Adventurecorps. They've got a Death Valley Century and Double Century during the spring and fall (no intense heat). I know a couple who has rode these a few times and said they are just beautiful rides.
Same company also sponsors the Badwater UltraMarathon (135mi run starting in Death Valley and finishing at the top of Mt. Whitney) and Furnace Creek 508 (bike ride). The 508 even has fixed-gear divisions so if riding 500+ miles thru the desert isn't hard enough you can limit yourself to one gear (and a steel frame). Brutal stuff.
"My actions are my only true belongings. I cannot escape the consequences of my actions. My actions are the ground upon which I stand." Thich Nhat Hanh
