Having no experience as a runner... since "middle distance" like 1600m is more just becoming at all out sprint it seems to me that a combination of intervals and metcon would be the most effective.
If you can transfer from your phosphagen to glycolytic to a bit of oxidative with very little dropoff, you'll do really well. Having just read what Ross said about his experience I think this would be the best idea to base it on.
Concerning HIIT like intervals I think it would be best to program variations of run/sprint vs. jog from 200/200 all the way up to about 350/50. I'm *not* actually sure it would be a good idea to do so but run maybe 2000m for your intervals.. but at the very least about 1200m. Also, I was reading some stuff earlier on Wariner and Johnson for the 400m and they were running a bunch of stuff shorter and then resting 1-5 mins and then running more. For example, 300m then 250m or 3x350. Something along the lines of 1200m then 800m with a minute or two rest inbetween might also work well.
Anyway, those are just my thoughts from a non-runner.
I'm almost positive that metcon is the way to go since the WR the 400m times people are looking at start to change energy pathways around 300m.
P.S. make sure your running form is very, very good.