I ride pretty long distances (100+ miles every Saturday or Sunday) and listen to audio books on my MP3 player. Check out the selection on Audible.com. Getting involved in a book really occupies your thoughts and takes your mind off the tedium. I also only EVER put in a single earbud - which is fine for a book (no stereo issues like in music). Put it in your right ear so your left ear is free to listen for the sound of approaching cars.

Good luck on your solo century!