You did a century, albeit a metric! Nothing wrong with that. IMO, a metric to about 70 miles is a good long ride. The miles between 70 and 100 are kind of like empty calories. They don't really do anything productive insofar as the ride is concerned other than make you tired, physically and mentally.
Having said that...
Go for it, just to say you did and to prove you can! That's about the best "explanation" of why we do it that I can come up with. Why? Just because I can.

And besides, those 60-70 mile rides are good training for centuries. Be a shame to just waste them.