Intervals on the W&OD Trail (7.75 miles)

I programmed my Garmin Forerunner with a custom workout.  One mile of warm-up followed by 8 intervals of .5 miles between 8:00 and 8:30 / mile followed by .25 miles of rest at 9:00-9:30 mile.  Then a short cool-down run.  It was fun, but I need to tweak the rest speed (lower) and tighten up the speed interval (there's a 30 second variance which is too big).  Here's the graph...