What are the normal long disance hurdles that occur? Aka, the stumbling blocks that generally act as a barrier to a cyclist? For example, a real bad one at that, would be the 20 mile marker for someone running a marathon.
I see your second post answers most of the first post's questions.
For the most part - if you are tough guy then you can ride a bike all day and "tough out" a long ride.
As far as the "next step" - things will get hazy. For the most part - being able to ride all night - and then still be able to ride well another day is about as far most people go. In theory - any healthy person should be able to ride a bicycle for 24 hours - but you know - TV and french fries and beer ruin it for most.