Agree with mpre53 and maydog - if you finished strong at 60+, you're going to make the 100.
Mentally you might want to think about dividing up the ride into 4 sections - 25miles or so each. That may coincide with the rest stops if it's a supported ride. You know you can comfortably do 25/30, so just ride 4 of those in your mind. Back off your normal 25 mile pace slightly since you're going to do multiples of 25. If you use a heart rate monitor, know your max heart rate and map out where the hills are in the ride. Give yourself some breathing room by keeping your heart rate at 80% or less of maximum before starting a large climb. Maybe 0.5 - 1.0 miles out from where you know a climb will start, back it down slightly, focus on efficient breathing, and shake off any lingering fatigue. When you get to the hill(s) spin it (them) strong and steady. Look for hills in your training area and add them into your training rides for the next two weeks. Develop a rhythm that suits you for climbing - experiment.
Riding 3x-4x mid week for the next 2.5 weeks will help. If daylight hours are too short, find a trainer and ride inside. "Taper" the week of Oct 18-25. No major strenuous rides - push enough for a cardio workout, but not so much that you feel the need for extended recovery time.
If you're going to play with food, do it now. Many people don't respond well to changes in food intake when their body is heavily stressed. Weather and how hard you ride can have a significant impact. If you really push in temps warmer than you're accustomed to, then eat significant solids, you may find your stomach doesn't handle that well. Not all Fall days are cool and comfortable. Also be careful with the length of your stops at rest stops (if available) - too long and your muscles will probably start to tighten up. After 15-20 minutes it's basically like starting all over again. If it's a no-stop 100, take stock every 20 miles or so and make sure you're hydrating and taking in some carbs - especially after 50 so you don't run out of gas in the last 10-20 miles. Have a great ride!