the guy who said run more, more often, and run slower and then you will run faster. speed with no enough base is the killer. 10-15 of your mile at the MOST can be above z2. I have won my AG in several IM's with 10-12 miles total above z2 in the 12 week build up the race. Z2 is not easy when you are good at it.

Be patient and run 5x a week at a z2 HR, test yourself with 5 miles on a TM or track at Z2. Until you top progressing on that test, don't even bother with any speed. And at that point it does not take much.