Back in the day before Str*va I tracked every ride in Excel, copied the data manually over from bike 'computer' and made fancy graphs - this one covers a 10 year period from 2004 to 2014. Things to note:
It took me a year to go from 14mph average to 21mph average.
2009-2012 there is a step up in speed - this is also when I started to ride with racers on the weekends and evenings which increased my fitness.
My new route in Nov-2011 resulted in slow down in average speed