To date 104 in one sitting is the most I've done, but I figure if I can do 104 miles then I can pull off 200 miles.Right?
Wrong.
It is true that if you have the background fitness to ride a 100, you can most likely keep riding for two hundred. However, to complete two-hundred mile ride will require that you ride the first 100 miles without developing any soreness nor significant fatigue.
Simply finishing a single 100 mile ride is hardly evidence that will ever make 200. If you mean to say you have finished a 100 mile ride with no pain and little fatigue - then great. Good Luck.