Importance of fast repeats greater than 400m?

Build an aerobic base.

You use the example of 8x400 and 16x200. Even if the 16x200 was with say 45 seconds rest and the 8x400 was with 90s rest, I'd expect the 8x400 to be harder for everybody, even though to the uninitiated it might seem to be the "same" workout.

To make the point, extend it out. Let's say you did that workout at mile pace. The next "doubling" of that workout would be 4x800 with 3 minutes rest, which would probably be extremely hard. Next would be 2x1600 with 6 minutes rest at mile pace, which would be impossible.

Basically as you go longer, you're using up your CP stores. At a certain distance you will start to "fall off the cliff" and you won't be able to effectively recover, even with increasing recovery times.

this here wrote:

Build an aerobic base.

I have an aerobic base. Any opinions on the question itself?

If mile is your focus, you will want to work on extending your anaerobic endurance so yes, you need to do longer intervals. Example, instead of doing something like 10x200 at mile pace, do something like 4x500 at mile pace with 3mins rest; same amount of volume, but tougher workout..

You wanna teach your body to continue operating at pace while you are losing the lactate battle. (buffering)

That said, I'd make that its own session, and make the 400m pace work its own session.

Fast but not good wrote:

maybe the reason is because fast-twitch fibers last about ~40 seconds, so in the 300s I can rely almost entirely on fast fibers, but in a 400 or 500 the amount of slow twitch fibers needed greatly increases? Let's say for the mile, should I focus more on repeats of 500m-600m, in order to train myself to run fast off of slow twitch fibers?

Not just maybe.