There was an article in Bicycling Magazine recently about Jure Robick (sp?), a member of some obscure European racing team, who unofficially cycled 500 miles in 24 hours. He has won the Race Across America 2-3 times. The title of the article was something like: "That which makes me stronger makes me stranger."
My longest ride was 125 miles, when I participated in a road race from Mexicali to San Felipe, Mexico on the Baja. Took me about 7 hours. This was 25 years ago, when I was around 40. I am still riding the same bike -- a Univega.