50k doesn't sound so bad....as long as you don't do the math.
I have run the 10 Mile Trail Challenge at Salt Fork State Park several times. It is a fun and hilly trail run. The Bigfoot is three loops of the same trail. I knew about Bigfoot but always questioned the sanity of those wanting to run three loops. One is hard enough. But here I was running three loops that would provide me of 4,500 feet elevation gain and almost 7 hours of "fun".
The first two laps felt pretty good. I was fueling up and keeping a nice steady pace. The third lap started to wear on me as the energy in my legs just wasn't there. I wasn't to full bonk yet but a fourth lap would have done me in.
So I did 50k....or 31 miles. Not sure if I will go back to long distance trail running like this again but it was nice to do it.