I am retired military and now contracting overseas, recently ran a PT test for a new job which included a 1.5 mile test. What has worked for me for several years is this:
Hop on a treadmill, set it to the fastest speed you can maintain for a quarter mile and then run that distance. Hop off, rest 90 seconds, and repeat for a total of 4 rounds. When I started doing this several years ago, it got me down to a 9:30 1.5 mile run. I've done it off and on since, usually when I need the conditioning boost. Most recently I worked my rest period down till I was only resting 45 seconds between rounds. If you have access to a high school track, that makes a better alternative.
Also, when I first did it years ago I would perform a set of 8 front squats with 185 immediately before hopping on the treadmill.
I typically do this twice a week, interspersed with my lifting days. I am able to maintain size and strength using this routine - keeping a double BW squat and 1.5 BW bench (in addition to other lifts).