Thank you for all your comments. RAMROD was a great adventure

To answer some questions:

My bike is an 1988 Paramount converted to fixed gear. It fits me very well - this and the contact points: WTB SST saddle, Nitto Bar, and Crank Brothers Quattro pedals make it quite easy to ride for long periods of time.

I have no problems with my knees. I don't push a big gear.

Conditioning? Ride a lot. Long rides at least once a week. Commute 56 R/T several times a month - more when I can get away with it. I also think trail running helps too, especially up hills so you get used to being stressed physically. Doing a 12 mile climb out of the saddle feels more like running up a hill than riding to me. I am lucky enough to live in a fairly rural area so all this is easy for me (the long rides, trail runs).

One word: CHAMOIS. Anything over 30 miles on the regular need to get some cycling briefs or shorts that have a pad in 'em.

Next, the most natural gear helps greatly. Your body, legs, knees won't allow yourself to ride over 40 miles again if you have the wrong gear. Just like the OP said, "it feels more like running up a hill" meaning it's a sustainable natural gearing that allows him to ride 100+ miles on a fixed gear. Bravo!

I think OP did the whole thing fixed though because he was talking about braking a lot on descents?