The popularity of races longer than marathon distance has boom ed in recent years as more and more runners seek new challenges.
A 50K or 30 mile race is generally seen as the first rung on the ultram arathon ladder, and from there race distances are pretty much unlimited. While most races will see you covering a set distance as a goal, some will challenge you to run as far as you can in a given time period (usually 12 or 24 hours).