good advice from everyone here! In Jan 2005 I rode my first metric century ever... 3 weeks later I rode my first century with no problems at all... My personal feeling is that anything over 50 miles is mostly mental. If you're convinced you WILL do it... then have a great ride and be sure to tell us about it!
For me the most important thing on a century is pacing myself... I go out at the same pace I intend to maintain... and lordy... I can honestly say I am NOT a racer! LOL... not even close...
There is a fine line between "hobby" and "mental illness".