I have a single speed bike with a surly flip/flop rear hub. I have been running freewheeled for several months and loving it. I flipped the back wheel to fixed the other night and decided to give it a shot. I ended up riding 25 miles with some moderate hills and now I can barely move. lol. That's gotta be good for ya. I recommend moderation for the first few fixie rides.
I have a set of wheels that I built on the same surly hubs. Love them. I used black surly hubs, black mavic open pro rims, and black SS spokes. They look pretty sweet and the hubs are nice and smooth. Plush... I love them. Even thought I have a flip/flop rear, I have yet to put either a different fixie gear or a freewheel on the other side. Mine are on a Biancci Pista frame with a bunch of other random parts (mostly campy). What frame are you riding?
I came up with an old Cannondale Black-lightning frame and crank. It was a pretty high end road racing frame from the early 90's I believe. I built up a wheelset with the black surly hubs and some prety deep rims...They're not velocity deep-v's, but I wish they were. I like it a bunch. It's one of my favorite bikes to ride.
First weekend in August, I rode 27 miles fixed on Saturday and 23 more on Sunday on a home grown commuter with 48:17 gearing. This bike is soooo smooth it feels like buttah in the flats. Did a quick 6 miler last Sunday with my daughter, forgot my riding shoes, so I had to ride in my crocks.
Kentucky does tend to be pretty hot in the summer. The worst part is the humidity though. Texas heat didn't affect me as bad as the KY heat. I don't remember how long it took. There were some breaks in the ride. I seem to be able to average around 12-15 mph fixed and a little higher average on the freewheel side.