As I have been cycling, I have discovered that the temperature makes a difference. When it is hot, especially humid, it seems to be harder to pedal. Now that I am 30, perhaps my old joints mind the humidity ;P
15 min and 45 seconds, 5.21 miles, 19.3 avg mph, 31.9 max mph.
Granted it was downhill most of the way, but I am pretty happy that I averaged close to 20 mph!