i did the 2005 seattle to portland ride (200 miles) on a Bianchi Pista, but in two days. i toned down the gearing to 40x14, but my knees still hurt on the 2nd day.
the downside to a true fixed gear is that your knees really take a pounding, mostly from backpedaling/skidding (at least in my case, since i rarely use my hand-brake).
anyway, on the stp our first day was something like 7 AM - 3:30 PM (110 miles).
and the 2nd day was something like 7 AM - 2 PM or so. i'm guessing we held a 16-17 mph avg, somewhere in that range.