How to build average to good cardio in a relatively short time

I finally received my ship out date for Air Force Basic training, leaving on March 25th. I am a 17 year old male, ~140lbs and generally feel comfortable with my overall strength and other fitness aspects, but have always had embarrassingly bad cardio.

I need to be able to comfortably run 1.5 miles in 18:30, which by all accounts should be easy but I can barely manage around a 22 minute time, and most of that is spent running with a heart rate well above 140 bpm

I have been walking/running about 2 miles to the best of my ability about 3-5 days a week before weight training each day for 2 weeks now and do not feel like I have improved whatsoever.

Is there anything I should be doing differently, could be doing better or should be doing instead? Or should I just keep it up and wait longer to see improvements