Here's what happened to me. My average MPH was slowly progressing over the last couple years 13.4, 13.5. 13.6 then one day 14.2 Yes I hit 14! Well just the other day I went out for 20 miles, two 10 mile loops... The first 10 miles I did 15mph, then I said I think I can go faster and did the second 10 in 15.7 Now I'm thinking that before the end of the season I am going to do this loop in 16.
Early this year we would do 20 miles on Monday(rolling), 20 miles on Wednesday with hard hills and 15 miles on Friday with intervals. Don't know if that did it but something just changed for me.