I regularly do centuries, both supported and unsupported on my fixed gear. If the century is supported, a couple of water bottles, a couple of clif bars, and some power gels is fine for nutrition, you definitely still want an extra tube, pump, etc.

Unsupported I carry 2 water bottles and as soon as I'm down to 1/2 bottle I stop and fill them both. Also, you'll need to carry all of your food for the ride. I usually do ~20 calories per mile, so ~2000 calories.

Good luck! Centuries are a great way to kill a day on the bike.