My wife did her first metric, followed by a full century the following week a few Septembers ago after only starting to ride in May. Of course it's doable. You could probably do it now if you put your mind to it and could suffer some discomfort. Your training is what will enable you to do it faster and with less pain (both during and after the ride). Keep increasing your distance little by little until you can do a 50 mile ride non-stop (or even with some rest stops), a few weeks before your metric, and you'll do just fine. Remember to take rest and recovery days too...those are the days that make you stronger, believe it or not.
__________________
"Let us hope our weapons are never needed --but do not forget what the common people knew when they demanded the Bill of Rights: An armed citizenry is the first defense, the best defense, and the final defense against tyranny. If guns are outlawed, only the government will have guns. Only the police, the secret police, the military, the hired servants of our rulers. Only the government -- and a few outlaws. I intend to be among the outlaws" - Edward Abbey