In an interesting book 'Spartan Up' author De Sena has this take on going the distance...
'So instead of thinking, Ugh, I’ve got twenty-four miles to go, focus on making it to the next telephone pole in the distance. Whether you’re running twenty or one hundred and twenty miles at a time, the distance has to be tackled mentally and physically one mile at a time.
What's the next mini milestone for you to take on next?
Line it up. Take it on. Chunk it down into bite sizes.
You can do it. You know you can.
Your next 'telephone pole' is ahead.
You next step of potential is right in front of you.
You can reach it. You know you can.
Joe De Sena is the founder and owner of Spartan Race. He is also a world class endurance athlete. Check out spartan.com