I went on google (because I'm a google guy) and typed in "define stamina" and it gave me this definition: "The ability to sustain prolonged physical or mental effort." When it comes to running ultra distance races, I believe it isn't an 'or' but rather an 'and' because for one, it takes a lot of physical effort to run 50 plus miles, and two, it takes a lot of mental effort to continue to push your body for 50 plus miles. This isn't just something you can just one day come up and do. Stamina is built from the ground up, and it needs a lot of upkeep... especially physically.
I remember when I was first training for my marathon, I would do both my running and sports activities with my friends, forcing my body to push onward, increasing its stamina. Today, I felt so alive when I started this morning with a 2.33 mile run in 15:38 (6:43 average), went to work, then afterwords worked out with 20 pull ups, 50 push ups, and 30 minutes of cycling, followed by playing a little sand volleyball with the young adults group, finishing the day off with a pretty intense soccer game, where I played hard, but we couldn't win it. Our lifestyle really affects our stamina, and when left by itself, we lose it.
When we are running towards our goals, we need to have the stamina to do so. When we are achieving that goal, we need to be able to mentally push ourselves and hold on to it. It's because of stamina that in the last minutes of the soccer game, you can push yourself hard and score. It's because of stamina that people do all-nighters to finish their studying and ace a test. It's because of stamina that we are all given a better chance to reach our goals. We have to understand what stamina is and how helpful it can be. Once you come to an understanding of what stamina is, and how helpful it is to you and your life, you can focus on it and take steps towards adding to your stamina supply.
Here's the tip about how to keep or upgrade your stamina.
Very simple. Add more to your plate, little by little. When you know you're going to be able to take a break, then you can add a little more, but the most important and most simple step is to just keep yourself at a point where you're either adding more into your lifestyle, or just keeping a good lifestyle that supports you.