I rode 75 fixed last Sunday; I know, not a century, but was my longest fixed ride yet. I rode a 45/18 and was geared perfectly for gently rolling terrain here in N/W FL. I felt absolutely no leg discomfort and was surprised at how nothing except butt and wrists was any worse for the wear. I did find that I missed having shift/brake hoods somewhat as I ride on the hoods a lot with my geared bikes, hence the wrist discomfort, but no biggie. I suspect saddle fit is doubly important when doing lots of miles fixed simply because you can't/won't shift around on it as often as you would do when coasting on geared bike. Have a good ride!