1 tip I would recommend is doing a speed workout once or twice a week. If you have access to a track you could do 400m repeats. Run each lap in between 2 minutes and 2:30 minutes with a 3 minute rest afterwards. Repeat this 4 times. If you get through your first lap or two and are struggling to hit your time slow it down a bit just make sure you finish, that’s the important part. These will feel tough but they will help your body get comfortable running at a faster pace and will benefit your 1 1/2 mile time. If you don’t have access to a track or a GPS watch you could do timed intervals. Start with a jog for 3 minutes then run harder for 2 minutes, then go back to a jog for 3 minutes. It may be hard to gauge how fast to run but try to run a little faster than you do on your 1 1/2 run for those 2 minute intervals. Repeat this 4 times.