Granted, I am not doing Centuries on my fixed, but I do put some miles down.

My biggest "Eureka" with the bike was getting the bars just right for position. A fixie will never give you a break, and not being able to get a decent bar position for upper body support is a killer for me. I currently run relatively wide moustache bars with a stem 10mm shorter than normal (road bike). I can stretch out comfortably on rides in excess of 2 hours.

Even standing, you gotta crank it over. Just my observation.