Interesting chatter here! I'd be quite certain prosopis maxed out at a 5:58/mile pace when experimenting with some 400m repeats. Prosopis, as you learned, that's not the right pace for your training at this point. You did learn an important lesson - you're hurting in new areas, and that means those muscles are not used to being engaged in that manner.
That's the point. The interval training stretches and works the muscles differently ...you break them down, and they heal up stronger. Over time, the stronger muscles will allow you to speed up your longer runs or just run longer because it's less effort. So keep at it ...but not so fast yet!
jb - lots of advice above already. You could even just start with some 100 yard accelerations. This lets you avoid the turns, and you avoid the risk of slipping into bad form. As you accelerate, focus on the foot strike (avoid a heavy heel strike), keep the steps light and quick, don't overstride, do let the legs kick back, keep the arms at about a 90 degree angle, feel the arms rocking straight forward and back (not side to side across your body), relax the hands. Don't go into an all-out sprint. Just accelerate with a good stride and rhythm, then let it wind down. Walk back, and repeat a few/several times. You won't get overly winded, so you can stay focused. Just get the feel of good form. Then you can start to work that into 200m, 400m, or longer. As Sand notes, swimming is all about good form. But running has a lot of that too. Over time, the track work will translate to better pacing on all runs.
I agree with
Ned on the shorter rests ...I prefer that I stay a bit uncomfortable as I move through the repeats. Better to control the pace than to lengthen the rest.
Sand, yes, you're motivating me.

But I also have looked at the times I need to run to compete in this CARA race circuit here in Chicago. If I start the year sub-Sand (<20:29), I might be able to compete near the top of my age group. It's probably been twenty years since I put this kind of focus on the shorter races (and a run-only focus), so I'm eager to see what comes of it.