Well, it is all relative to what one trains for really. Lance said his first NYC marathon was the hardest thing he ever did but that was only because he didn't prepare well for it. The 70 year old can finish Ironman as long as he or she stays within their abilities as far as intensity. A time trial at full speed over 15 miles can possibly be the "hardest" single event for that matter.

I would say that the Ironman is an exceptional event though. I would actually like to see where Lance would finish if fully trained. Steve Larson made that transition a ways back and won an Ironman. Some of those Eco challenge events can be brutal too. Ultramarathoners are exceptional as well but it is more about endurance and staying within the limits rather than all out athleticism. That is where the Tour De France can be seen as one of the most difficult because of the endurance and intensity that is needed to compete. I have had this argument with friends who think cycling is easy and bikes do most of the work. LOL. Until I take them out on the road and have them try and keep pace.