I've used my lighter 'racing' bike - a carbon Bianchi 928 - on 300Ks and a 360K fleche. I've also used a titanium bike for some - the way it smooths out the road is greatly appreciated. 
More important question for me is gearing. I'm building up a bike with a triple, specifically for longer, hilly rides. The gear you can do a hill on a 50-mile ride is very different than when you hit that same hill 250 miles into a ride. I'm learning that I need to use much easier gears if I want my knees to hold up later in a ride. (DNF'ed a hilly 400K because my knees blew out.)
I put aerobars on my brevet bikes for a few reasons. One, having that alternate hand position and taking the pressure off my wrists/hands is nice on a long ride. Plus, they give me lots of places to hang things. The cue sheet can hang from them, leg and armwarmers can be removed and threaded on the bar's arms, lights and/or batteries can get strapped on there... nice stuff.
For 3 days, I get to part of a thousand other journeys.