Funnily enough my PR is exactly a 15:40. What I did to get to that point was semi-structured as I coach myself. Mileage was around 65-70mpw leading up to my peak races where I was closer to 55mpw.
I was doing a lot of longer tempos/fartlek early in the season. Like 20 min LT runs up to 30 min LT runs. I was doing things like 4x 3 min, 2 min, 1 min with 1 min rest @8k pace, 12x 90 sec on/60 off @5k pace. Then doing more traditional interval work stuff in the middle. 4x mile repeats, 8x 1000m, mixing it up some days. I generally ended all sessions with something like 4x200m @800m-mile pace. To develop and maintain speed.
I also began more speedy work @mile pace such as sets of 300-600m reps with moderate rest to get my body adapted to running fast.
Long runs were generally 12-15 miles, depending on the week. And most were alternating easy long run and progression type long runs every 2 weeks.
All of my easy runs were around 6:30-7:10 pace and recovery runs were more like 7:00-7:30 pace. I did 2 workouts with the long run generally a week and the rest were easy or recovery days. Usually with strides for the neuromuscular effects.
I recommend that you balance your schedule for what’s best for you and find the right mileage. Increase intensity as you see fit and when you get fit. Take easy days seriously and race a few times to get small PRs that will lead you to a big PR (your goal).
Good luck bud