I have recently become very interested in extreme endurance events. I am sure there are Guinness world records for any number of activities, but I am currently debating the standard triathlon sports of swimming, bicycling and running.
Swimming the English Channel is the most well known extreme swim challenge. In 2009, 29 individuals successfully completed a solo crossing. Historically less than 50% of swimmers succeed. While the distance is only 24 miles, it is not unusual to swim 30-40 miles given the tides and the currents. My longest open water swim is a mile.
Bicycling in the Race Across America is the toughest event I am aware of. The race covers 3000 miles on limited sleep and is completed in 9+ days. Last year 14 of 30 starters, finished the race. The longest I have ridden is in MS150 events which are 150 mile rides over 2 days.
Running the Badwater Ultramarathon is the toughest race I am aware of. The distance is 135 miles and the winner can finish in about 24 hours. The temperature extremes are absolutely amazing. The website even makes a point of racers who require intravenous fluids will be disqualified. That is pretty extreme. Interestingly the event is currently by invitation only and last year 75 of 86 starters finished the race. My longest race was the Austin marathon.
By comparison the 513 individuals reached the summit of Mount Everest in 2010 and the Kona Ironman field is limited to 1500 individuals due to high demand.
I should include K2 in the world of extreme challenges. First climbed in 1954 there have been 300 summits through 2009 and 77 fatalities. It will be interesting to see how the 11 expeditions fare this year on the Savage Mountain.