I did 4 x 400s at the with the same target pace as my 300s and the same 2 min rest.
400m warmup, stretches, drills
Saucony spikes on
4 x 400m - 68, 69, 75, 75 with 2 min rest between
200m - 28
I wasn't able to sustain the pace beyond the second rep. My target was 68. I must admit a slight cheat ... took an extra 30 sec rest before my last 400, my pulse was about 150 when I started the last 400m, 170+ when I finished. I rested and ran a quick 200m before calling it a day. My ankle hurt on the last 200m.
It hurt but not as much as the Sunday morning workout. I was a little heavier but better fueled today. Ate some carbs last night. 141.8 after workout.