What took you so long
Seriously - brilliant running. That is the type of run many of us can only dream of!
Me too it's only the last six months that I've been able to sustain 7 minute mile pace for anything above 5k. 12 months ago 8 minute mile pace was a struggle above 10k.
Speed work has been the key for me, get yourself out of your comfort zone and when you go back to longer runs a pace that seemed tough before, all of a sudden feels very comfortable.
5k's at park run highly recommended to those who haven't done it, they really push you to run hard and fast.