12 X 400m A new approach

So I finally got out to the track today for a workout with the High Plains Harriers (Local running club).  The workout was 12 X 400 at 73-75 secs with about 90 sec rest.  Now in comparison to workouts in the past this is way slow for 400s, but I have been thinking abou the benefits of running slightly slower 400s.  Couple of things were immediate, why run faster than 70 secs if in races 5k and up I am not ready to do that the whole way.  What I need is to be able to run at a fast and efficient clip, that is sustainable.  So with the workout today, the slow pace allowed me to really concentrate on my form, good turnover, foot strike, slight lean, working the arms etc.  but it wasn't so fast that I lost control of my breathing.  As I get in better shape I hope to drop these to 68-70, but that will come.  While this was an "easy" workout my muscles are sore.  This tells me that I worked the system I wanted which was the leg muscles and not so much the cardiovascular system (like a tempo or longer repeats would).  Kind of like lifting weights if I can lift 50lbs fairly easy lifting 40lbs should feel good as well.  Anyway that is my theory at the moment.  I want that fast form not to feel awkward or like I am pushing to much, I want it to be comfortable and feel powerful.
  The splits were 75, 75, 74, 74, 74, 74, 74, 73, 74, 73, 70, 71,  I averaged 90 seconds rest but I wasn't super strict.  But again I wasn't working the breathing system just the legs.  I was pretty happy with the workout especially as I felt I could push a bit faster.  But definitely did what I wanted too.  I am hoping that this type of workout will springboard into longer interval training 1200s and 1500s, so I can run those at a descent clip as well.