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If you can run a marathon you can do an ultra. It's the mental endurance that I find the toughest part - having the drive going for hours and hours can be tough. Also women seem to do well at the ultra distance. Find a nice relaxed timed event (that way you only have to think about just getting the miles) and give it a try. Just think about completing it for the experience, don't worry about a specific time/distance. Even a DNF will help you learn and other ultra runners are a helpful group.