When doing easy running, all you need to do is get your heart rate in the zone to receive aerobic benefit. Those zone is usually 140-160 beats per minute. The pace needed to sustain the heart rate will get quicker as you get into peak shape, this is why pace isn't important.
On your next run start off like you normally would, and 2 miles into the run, stop for 6 seconds and count your pulse, multiply by 10 (stick a zero on the end of the number) and that's your heart rate. Pace is not important, if it is 90 degrees out, you need to slow down from your 6:30 pace or whatever, if it's 55-60 you can probably maintain that. Just run with your heart rate. If your heart rate is above this threshold you should slow down because A. Running faster on easy days produces essentially the same aerobic benefit as running slower with your heart rate in the zone B. Running faster on easy days adds un-needed stress to the skeletal-muscular system. C. You will recover less quickly if you run your easy days too fast.
Also, you don't need to run fast on easy runs, look at Ben True's strava account, he does easy running at the pace you describe and sometimes slower, and his 5k PR is 3 and a half minutes better than yours. I also know that the top 5 guys on my college D3 XC team freshman year (went to nationals) were all sub 15:30 and did the vast majority of their easy running at 7:00 pace or slower. I say vast majority because the end of long runs would turn into a tempo run. All recovery days and filler easy mileage were 7:00 per mile or slower.
Slow down on your easy days!