congrats are in order! good stuff!
i just got through a ~42 miler with a group of geared folks on sunday. the one thing that i noticed is that, when your legs are tired, downhills are a bit rough. i probably had another 10-15 miles left in my legs when we finished the ride, but i was happy to finish up. a century on the fixie seems daunting.
when i train for centuries on my geared bike, i look at being able to complete a 75 miler as a training ride at about 20-22 average mph as enough to finish a century. add in drafting and just the joy of riding with others (along with some pit stops with food/water), and the other 25 miles go pretty quickly.
as everyone else has said - hydrate a bunch. also eat.
just for fun, i try to do hillwork at least once a week. keep attacking the same hill a few times after and before a ride -- you'll be amazed at how much better you get over time.
last but not least -- find a training partner that's a bit better than you are. i have a hammerhead of a friend that just attacks constantly. it makes me train harder, which makes those long rides seem easier by comparison.
ride on!