What kind of base have you been riding a week. What has been your longest ride over the past several weeks. Have you ever ridden a century on a geared bike? Assuming you are young you can ride 10 miles 10 times or whatever the # of rest stops buy you would not have "fun". You need to be fit enough it's challenging but fun. I'm planning of riding a century the first Saturday in October on the fixed gear bike if the wind isn't blowing hard. I've built my long fix gear ride to over 5 hours and will do a couple of 5:30 rides and then taper for the century. But if your question is "can I just jump on my bike and do a century fixed with no real base and be ready for the century in a month?", I recommend you spend months building your base and doing shorter rides first-like a half century. And don't make the mistake that a flat century is easier than one with lots of rollers.