It's never fifty. It's never one hundred. It's always eighty.
The first eighty pages are the toughest for me. They take the longest to write, and they require the most false starts.
You are introducing characters, weaving in backstory while moving newborns through the plot. For some reason it always seems to take me about eighty pages to get most of the backstory down and get the players introduced.
It's also a little like that period when you first meet somebody and start hanging out. You have an idea of what this new friend is like, but that perception can drastically change over the next several months. This is also why I don't get too hung up on those first eighty pages because I know the characters will grow as I write. Once the book is done I'll go back to those pages and realize the character will no longer respond the way the scene was written because I know that person so much better now. When I wrote those early scenes, we'd just met. Now we've been living together for at least six months. Maybe much longer. I don't have a solid grasp of my characters until I've watched them in a few scenes. Until we've hung out a while.