I agree that if you can comfortably ride a metric century, an imperial century isn't significantly harder (it just takes longer). I rode 100 km the week before I rode my first century (actually 111 miles altogether) and didn't have any showstopping problems with fitness or mindset.
Someone mentioned a emergency repair kit with a chain link a few posts back. I broke a chain halfway through a 100 km ride and had to call a family member to get picked up; very demoralizing and embarrassing. From now on I carry a small multitool with a chain tool (the Park Tool IB-2) and a couple of KMC masterlinks that match my chain, in addition to everything required to change a flat (replace the tube and patch it later at home).