Thoughts about pace oriented training

I have switched over to pace training instead of heart rate training in July. I have never achieved my marathon time predicted by my 5K and 10K times. I think this is because I did not run fast enough on my easy and long runs. I have started training at the Daniels's Vdot of 42. The critical numbers are a 5K time of 23:09, a pace of 9:27 for easy/long, and a time of 1:42 for 400m repeats. I am also running 5K worth of .2-.25 mi intervals at 5K pace (7:27) and 5K tempos at half marathon pace (8:08). I have achieved all these goals and plan on staying at this Vdot until the Indian Summer half marathon on Sept 13. This should result in a 1:46:27. I feel more fit now that I have increased my E/L pace with the Garmin 205 GPS. I suspect I was running E/L at 10-11 min/mi while training for the Steamboat marathon. I am having difficulty running at 9:27. Today I ran an easy 5K at 9:01. I wanted to slow down to 9:27 because I have a slightly strained right Achilles, but I just could not slow down, even when the Achilles began to throb at about 4K. So now my plan is to run 5K's at 9:27 until my Achilles heals.