How much continual fixed gear seat-time do you have on the bike? Hopefully hours back to back in terrain similar to what you are going to ride.
I've found riding FG centuries to be
relentless.
Hour after hour w/ not a single moment to coast and ease over rough surfaces, adjust one's shorts or swap empty bottles for full are minor annoyances you'd never think of on a FW machine but all add up.
As @
79pmooney notes fit on a LD/FG machine, and adaption to it are critical.
With no option but to put out the spin and/or grunt on demand as terrain and wind conditions dictate physical stress can be considerable, a well fitted machine with lots of seat time on it is my recipe for century riding.
I use an old road race bike converted to FG w/ two brakes, a machine designed for riding at pace on open public roads for considerable distances. Two bottle cages and two bottles in jersey pockets give good range in the heat. A pump and full flats kit w/ a wrench to remove the rear wheel, front is QR, are carried w/ cell phone and ID/cash/CC/insurance and light food. I know my routes and the distances to service but start early in the heat and pace myself.
Have fun, LD/FG riding isn't for everyone but I enjoy the challenge and it's being part of a long tradition in cycling.
-Bandera