i think increacing your distance from 100km+ is a good launching point. what i did was start w/ 200kms and ride at 25lm/hr no matter what (not pushing) forcing myself to eat regularly and drink regularly. and get off the bike and stretch every once in a while. obviously having a spedometer helps. in a nutshell i would say just ramp up your distance every week until you reach a weekly total of about 300 km. the week before do a 200km or so ride and take a rest day the day following. you shoud be good to go. i'm always amazed at what the human body can do if you just push it a little and don't take no for an answer...

i would personally try some longer rides (like a 200 km brevet as Wes said) before i would attempt a double century. But, from my experience, a long ride on a fixie is no more diffucult than a long ride on a road bike (i've done 200km+ rides on both). in fact, long miles on a fixie help keep your legs from cramping as they have no time to sit idle and build up lactic acid.