I do both, cycling and running. But neither in a competitive or boundary pushing way. I do two half marathons a year with a mentoring program for youth. In the offseason, I like to get out on the back county roads alone and enjoy the ride, usually 20 to 40 miles. I've never considered which is harder lest I convince myself that one is too hard.
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"It is the unknown around the corner that turns my wheels." -- Heinz Stücke