I've ridden a few centuries on my fixed gear, as well as multiple double centuries and a 1200k on my road bike. First and foremost, you need to do address nutrition and hydration for any ride longer then 60 miles. Velocio's advice to eat before you are hungry and drink before you are thirsty hold true. Adding a fixed gear to the mix introduces a new dimension to the ride. Because you cannot coast, especially on downhills, metabolically a fixed century is more like 130 miles. The second challenge is hills. Going up is hard, going down is harder. Mke sure that your brakes a working well. Setting a drag break on downhills will save your legs. Third challenge is chaffing. Bag balm and chamois creme is your friend, and be sure to bring some extra in your seat pack. And above all, keep a positive attitude and you will do great.