I've ridden centuries on plain steel road bikes (a looong time ago)... or are you just more concerned about your injury and it's impact on what you think you can ride?
Have you considered some home PT for this? I have read that some think rolling is great for ITB problems - I used rolling on my calves for a few weeks with terrific results, both for plantar tendon pain and for calf pain. Different problem, but great affect. And you don't need much equipment. I just used a piece of PVC pipe.
Yeah, I am confident of my ability to train for and handle a century in terms of stamina, but not with my knees acting up after 20 miles or so. Home PT makes sense and could be a component of how I approach bigger rides, but will not fix the underlying disfunction which is bike size/fit and possibly my pedling technique.