Could give that a try. Seems like that would mean around 7 minutes for the first two and try around 6:30 for the last? Then see what's left for the final .1? From what I've heard the first mile of this course can be tough- pretty hilly- and the second mile easier.
I've primarily used some workout formats from things I've read from Pete Magill and he seemed to think those reps at the lower distances were good leading into races- while the reps with the longer distances (1000M e.g.) were useful earlier in the training. Hope I'm getting this right. In any case after this race I'm planning to do a few workouts (or at least one) at those longer distances again before the next race. In my case I think those are more useful as I can run pretty good 200's and 400's all day but not so much with the 1000M reps. Seems to mirror the struggles I have in races.
Do you think it is that you have inherently slowed so much or have you not been able to train as much? Your times still seem pretty good. Finding the time and energy to train properly is a challenge for everyone no doubt as work life and so many other things take precedence of course. Mid 18's for the 60's is pretty damn good as far as I have seen. Best of luck in the coming races.