August 400 m Repeats

I run 12 x 400 at Pott's Field once a week. The workout starts with a 1.5 mi warm-up run from my house to Potts. Then I run three one mile groups with a water break in between. The first mile is warm up laps in lane 2, then a water break, then I go for my best time on laps #5-8 in lane 1, then a water break, then I work on form for the last mile in lane 2. Times for lap 5 and the average of laps 5-8 are listed below.
Date Best BHR Avg AHR Avg/AHR
7/31 1:32 152 1:35 157 .0102
8/10 1:32 144 1:39 147 .0112
8/17 1:31 146 1:34 150 .0105
8/31 1:32 136 1:38 137 .0118