I agree with the ice. I would also look into shoes that fit you better and possibly look into your shoe/pedal interface. Are you using clipless? I know that too much float or a bad cleat position can cause achilles pain. You want to keep your pedal stroke nice and smooth without side-to-side movement.

As for the century, I say dream big. Most people who do at least 70 miles with similar terrain to their century beforehand can then handle the century. But don't push yourself if you're in pain. See how your body feels and consider all of this part of the learning process-- something we all have to go through!