Over-Under Speedwork

My coach has me doing weekly strength running, something I've never done before.  Wowzers.

Today's workout:
10 min warmup z2
5 x [5 min upper z4 (10:00 pace) then 5 min lower z4 (10:40 pace).
Cooldown

Considering last Friday was the first time I've ever run 5 miles straight, and that was in z2, I thought this was a bit wackadoodle.  So, as a compromise, I pushed on the upper z4 runs (even hit 9:30 a few times), then ran the lower z4 for 2.5 mins, walked for 1.5 min (drinking water), and ran the last 1 min of the interval.  Then started it all over again.  I did run all of the final lower z4 interval.  I probably could have run more, but give then this was 50 minutes of continuous HARD running, I didn't want to push it.  Still managed to get in 5.63 miles.

Now my challenge is not falling asleep under my desk.  I'm wiped.