i did philly to pittsburgh in 3 days a month or so back with a guy who was riding fixed. he kicked my @ss and was ahead for most of the ride (except the really big DH's).

we did 3 days of 190-100-110 miles. so it is definitely possible. it was pretty freakin hilly too!

but i rode singlespeed and was really happy to be able to coast, especially on the big downs.