IronMan Triathlon
2.4 mile swim (3.86km)
112 mile bike (180.25km)
Marathon 26.2mile (42.2km)
Must be completed within 17hrs (Starts at 7am, must be completed by midnight)
Any one of the above events is extremely physically demanding, and requires months of training.
With respect to all of you who have given examples.... All of the examples given above, are examining the cream of the crop, and saying "Nobody off the street couple compete with these guys" That's true, but take any group of regular people, or people who have the required skill (Driving a high performance car for example), and ask them to compete it COULD in theory be done. I mean, Richard Hammond from Top Gear UK did 2 laps.... no real training, he's just a pretty good driver, with some limited racing experience.
Ask me to go 12 rounds at 3min per round MMA with by best friend, I COULD do it, by the end, we might be on our knees throwing cheap shots, but we'll finish.
I've never ridden a dirt bike, but there many many people who can do it, with varying degrees of success.
I play Starcraft, (not Korean league though, those guys are dedicated), and I've done 4 (short length) triathlons. I could probably never bike 180km, nor run a marathon, I'm not built for it.
Ironman Triathlon? There are a very select portion of the global population who have the physique, mental endurance, and training to even finish.