I would suggest the use of an electrolyte replacement drink such as Gatorade while doing your century. However, I also suggest that you experiment with drinks and eating while doing your training rides to find out want works for you and what doesn't. Some people have problems drinking Gatorade full strength and dilute it with water.
For me, I take two large water bottles, one with Gatorade and one with plain water and I alternate taking sips from them every 15 minutes. Usually, they are used up around the 60 mile mark and I replace them. Regarding the "carbing" up business, I do not load up with carbohydrates before or during the ride, I just eat normally. During the ride I will eat a small PB&J sandwich around the 50 mile area which seems to do me fine. I hope this helps. Good luck on your 1st century.
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