I ride fixed most of the time but a Single Speed Chunker in the winter.
I would say gear the Single Speed about 2-3 gear inches lower than
the fixie. I like the fixie better but I really do enjoy bombing down big
hills (a lot of them aroung here) and regrouping.
I am part of a Fixie mailing list that a few people on it did RAGBRAI. There
stories were great !
I dont see any reason why you couldnt do a 70 mile day with no problem at
all if you were geared right.