I did read all the above comments but mine should stand for itself.

Before my first century the longest ride I did was 50 miles, same with my GF.
We were both fine, we both have athletic bodies, individual results may vary.

Nutrition wise we started trying different nutrition bars a month before our century so we knew what was edible and what worked for our bodies. (if you don't like powerbars try fruit based bars thats what we chose)

Gear for an unsupported century:
food, (I keep about 2 bars for every 3 hours)
water, (1.5L -2L)
emergency materials, (phone, cash, ID, rainwear)
Bike gear, (patch kit or spare tube/tire, small tools)

And lastly, I would not do your first century alone, or the only thing keeping you going is your will to hit 100 and that will make you not look forward to doing it in the future. Ride with a friend because it is around 6 hours and the last 3 will be the same as the first if you are doing an out-and-back.