I've found that more often than not, average speed is more determined by where and when I ride than it is physical ability. Throw in a couple of stop signs/lights on a route, and my avg. speed will drop 1-2mph over the course of the ride. I've ridden the exact same route multiple times, and had average speeds anywhere from 13.8mph to 17.2mph.
Which is why I pretty much ignore average speed as a useful metric.
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