I'm as old school as it comes, but, go try some new bikes- Specialized roubaix, etc.
1. If the cannondale doesn't fit, it never will. (edit- I shouldn't have said that- with enough tinkering, it might- but consider new tires, tubes, bars, stem, stem adapter- you'll be in hundreds at least. I've got a whole drawer full of misfit stems, so it's easy to go grab one, if you are starting from scratch, it's more hit/miss.)
2. I've done some retro builds on vintage road bike frames, and they are fun, but not as fast or comfortable as some of the newer frame designs. The newer frame designs let you have your cake and eat it too. Faster, more responsive frames, AND more comfortable. Specially for a 100 miler. You need something you can get on and get out and ride on, not tinker with.
Even if you don't get a new bike, try some out. I generally try to avoid the 'next big thing(ie- gimmick)' in bikes, they are just two wheels and a frame after all, but, things really have come a long way. When I went from a 1980's viner to a 2000's fondriest, I was in heaven (and this being said from someone who still uses a flip phone. )
