Definition from Bicycling Dictionary:
Randonneur – a cyclist who participates in rides from 100 to 1,200K (60 -750 miles), who enjoy the endurance and self sufficiency of these rides. They are often noted for being slightly different, not normal, off-kilter, needing psychological help, or just plain wacko. Statements like “I’m only getting warmed up after 5 hours of riding” or “When you see the sun come up on the second day of riding.” and that is the same ride. See photo below of a randonneur who fits this description.