As cited earlier, there are some good reasons why you might want to try. Sure, someone else has probably already done it, but by reaching beyond what you think you are capable of, you might learn a lot about yourself. What you need to know if you are planning to test your own endurance limits I talked with Karl King, a respected and knowledgeable expert in the field of ultrarunning and nutrition for long runs. Marathon training toughens the legs and enhances glycogen storage, but doesn't do much to improve the reaction of the endocrine system to prolonged stress. So, the typical marathoner is close to being able to run an ultra but need to do some longer training runs of 20-25 miles to stress the endocrine system, prompting it to grow and respond to ultra marathon conditions.
Typical marathoners walk as little as possible in their runs.
Ultrarunners need to intersperse their running with 4-10 minute walking breaks, especially when running a hilly course. Ultra runners commonly walk the uphills in a race. Marathon runners need to add short walking breaks to their long runs. Having done that, any runner who is fit enough to run marathons can do an ultra. It would be wise to start with a 50K, and then advance to a 50 mile ultra.