You really need to work up your distances -- say 35-40mi/ride, minimum. Time-wise, 2-1/2hrs to ride 40 miles maximum -- Or if you prefer, a 15-16mph riding pace, not including rest stop times. Otherwise, you're out there too long!!!
100 miles at 12mph is 8hrs of saddle time! 16mph and you are at 6hrs! Remember, that is saddle time, not total time. Traffic stops add up, too! Adding two scheduled 10-minute rest stops and one 20-minute lunch stop (plus one 'bio-break' stop) and there goes another hour. Mid-September is just 12hrs of daylight. Not sure when your ride starts, but keep that in mind!