I was wondering this myself. I came to fixed from a road bike background and was used to doing really long rides. I was thinking about getting a light, aluminum/CF fixie to take on long rides instead of the road bike, but ultimately decided against it.

I still think I am going to try and do 100 mile rides on the fixed, but road bikes will always be better for that purpose. You can go up steeper hills, go faster on the flats, coast, and have more control over how much energy you expend. I think it's REALLY cool that you are looking into this, and I admire your ambition. I think I am giong to try and do some 80-120 mile rides on the fixed, but will probably stick with the road bike on those kind of rides for the most part.

Good luck!

P.S.---you must be in really good shape if you're doing those rides.