Improvement?

I did my tempo run this evening, which is 1600m warm up, 2X1600m@06:44, 1600 rest, 2x1600@06:44 and a cooldown. The first two tempo miles passed at 06:42 and 06:33. I felt fairly fatigued at this point. The rest period was 1600m, which is quite long I think. The second set of tempo miles were 06:28 and 06:33 and I was surprised at how much I had recovered and I might need to reconsider my target pave of 06:44 or reduce the recovery period.