My last century was 4:45min in over 100 degree heat. I'm pretty close to BlazingPedals in speed. That same century (the only one I've been clocked on) my average for the first 60 miles was 22.7, and it dropped off after that. One that same ride, there was one stretch of brans new pavement, totally flat, where my 10-mile average was 33mph.

I was shooting for time though so that's not my "normal" pace - just what I am capable of. I'll usually cruise in the 18 range - that tends to be fairly easy in Texas.