Ted, I’m confident, but not positive that flat intervals are superior than hill repeats. The empirical data shows that high intensity intervals performed on a track can dramatically improve performance. I’m not aware of any empirical data that hill repeats do the same. Intuitively, we know that hill repeats force high intensity, but as long as you have the discipline, you don’t need hills.
So, as a coach I’m faced with a known (high intensity on flats), an unknown (hill repeats) anecdotal #1 (the best runners and coaches in the world use at least some hill repeats, including 80/20) and anecdotal #2 (my athletes were more likely to get injured doing hill repeats). So, balancing that all out, I choose to limit hill repeats.
I think the compromise is: do some of the intervals specified in your plan in the hills. Then you get both.