I regularly do 40-60 mile days on my fixie and I don't see any issue with doing a century. It's all what you train for and how you prefer to ride. I have done 10-12 centuries on a mountainbike over the years and had a blast on every one. Of course I don't imagine that I will finish anywhere near the front, but thats not what I plan to do anyway. Generally those rides are just fun rides and they seldom fail to deliver.