not the best runner, but i ran a 4:19 mile in school, and a 4:03 post school..
if your goal is to be faster on a 1.5 mile route, dont just keep running that course and try to better it each run, thats crazy. you're doing a " max " 1.5 mile run each time you run.. terrible advice..
best thing you can do in my opinion, is fartlek type training 1-2 days a week, and a long slow day once a week.. on your slow longer day,try to run 2-3 miles on the grass..
wish i knew more about how long you been running, your past running injuries, your sports background, and the time you have to train before this event..
you may be fit from other sports, but this dosent carry over to the specific muscles you will be using to run..
best advice i got when i gave up running when i was 35, was from a really old guy that owned a bike shop. past national champ.. he said if i planned on taking up cycle racing, train ez and slow the first year.. he said i may be super fit from running, but that i was running fit. not cycling fit .. different muscle movements.. great advice as i'm still racing the pro 1-2 races at 56 years old..
trying to be faster in a short period of time is a bad way to train, with injuries just around the corner.. sure hope you stay injury free.
good luck in this test
all the best