So you're still deciding if you want to tackle the ultramarthon distance. You're intrigued by the idea, but you have doubts. What exactly does it take to run ultras? As it turns out, it's probably not as difficult as one would imagine.
First, it does take some degree of physical fitness. If your goal is to simply finish the race, it is critical that you have the ability to spend a long period of time on your feet. The "time on feet" will likely be a combination of running and walking. How will you know you are ready? I like to use a 50% guideline. If you can estimate your finish time in the planned ultra, your training should allow you to spend at least 50% of that time on a single long run. For example, let's say you are planning a 50 mile run. You anticipate finishing in 10 hours. You could probably survive the distance if you are able to do a training run of at least five hours. Greater fitness will obviously increase the chances of finishing, but the 50% guideline is useful to determine minimal readiness.
Second, completing an ultra takes training. I do have a good friend that attempted a 50 mile race with a single 5k as his only training. He made it to about 27 miles before he DNFed (did not finish.) That was foolish. Brave, but foolish. In my opinion, one can get by with only a few runs per week and still finish an ultra as long as one of the runs is a long run of ever-increasing distance. Since the guide is for the lazy runners like me, I can admit to rarely running more than three or four days per week.
Third, ultrarunners need to be reasonably familiar with the issues they may face when running very long distances. They must be aware of the signs and symptoms of problems and know the appropriate response. In marathon-and-shorter races, most runners can simply run. If an issue arises, you can gut it out to the finish. In an ultra, that is usually impossible. It's awfully hard to gut out a chafed groin for eight hours.
Fourth, ultrarunners need to be mentally tough. You will experience some degree of pain. In all likelihood, you will experience A LOT of pain. In my first 100 mile attempt that ultimately resulted in a DNF at mile 65, I seriously considered diving on rocks to break an arm just to end the suffering of having to continue. Luckily I was too scared... instead I just let the cutoff times catch me and was mercifully pulled from the course. My problem was simple- I hadn't developed very good strategies for dealing with the pain. The second attempt hurt a lot, too, but I practiced much better pain-management strategies.
These are what I would consider to be the absolute minimum elements to running an ultra. Other things like prior racing experience, outdoor survival skills, an uncanny ability to navigate through the wilderness, being especially athletically gifted, or single with no children will certainly help. They are not necessities, though. Almost everyone has the ability to run ultras... even those that may not believe it today. If you can reasonably master these four elements, you will be in an excellent position to conquer the ultramarathon distances!