Reddit mentions of Relentless Forward Progress: A Guide to Running Ultramarathons

https://www.amazon.com/Koerners-Field-Guide-Ultrarunning-Ultramarathon-ebook/dp/B00MYEQGFI

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https://www.amazon.com/Relentless-Forward-Progress-Running-Ultramarathons-ebook/dp/B004WE78IC

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Both books come highly recommended. Also search for the science of ultra podcast.

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Good effort on this one. If you want to do a long run like that you pretty much need to train by running. Not fast running, just getting out and logging miles. Hiking (i.e., walking) is good, too, but if it's intended as training for ultras you want to keep your heart rate up there quite a bit, which you can do while hiking up ("power hiking"), but not really if you're just hiking on flat or downhill. Plus you have to get your body used to the pounding of running; it's much higher impact than hiking and you need to slowly build your running body up, otherwise you'll get overuse injuries.

There are good resources out there on how to train, e.g.,:

http://www.irunfar.com/2007/10/training-for-your-first-ultra.html

and some good books:

https://www.amazon.com/Running-Your-First-Ultra-Customizable-ebook/dp/B00VE731IG

https://www.amazon.com/Relentless-Forward-Progress-Running-Ultramarathons-ebook/dp/B004WE78IC

https://www.amazon.com/Koerners-Field-Guide-Ultrarunning-Ultramarathon-ebook/dp/B00MYEQGFI

I started at 46, relatively fit but a complete non-runner. I ran a 70K ultra less than a year later and now I'm training for a 100K. Consistent training and injury avoidance are crucial. It helps that I don't care much about my speed, so I'm content to do most of my training at an easy pace. But I have no trouble keeping up with friends in the local trail running club.

Edit: Check out Hal Koerner's new book. I can also recommend Relentless Forward Progress for training advice and Running Through the Wall for inspiration.