I remember trying to figure out how much harder 100 miles were going to be than the 60 and 80 I had already accomplished. Was it going to be "well, you just keep going a little further" or "After you've pushed as long as you ever did before, you have to get out and do another 30 miles" ?
By watching the pace it was much more like the former, with saddle time being an issue but a little less so for me 'cause my hybrid's pretty comfy.