How much have I lost?

Right now you have 0 fitness. That much time off will not show mercy upon return. However, earning that fitness back will be somewhat easier due to muscle memory. It will still take a lot of effort no doubt, but with consistent, smart training you will be back to 16 min shape within a year. The first 3 months will be a re-adjustment period. Running may not feel great but you will see fitness come back rapidly towards the end of this period. 3-6 months you will feel like a runner again. Fitness will continue to build with ease. 6-9 the fitness will begin to plateau as you near your previous top-level. You will have to work harder in workouts to keep the trend going forward. 9-12 months is when you will be able to build that sub 16 fitness.

This sounds about right.

Don't worry about what you lost. Just start running, listen to your body and move forward. Consistency is the key.

First month or so might be tough, but if you build back to your old training then it should take no longer than 12 months to get back to PR shape. December 2022 is really not that long ago and when you have already ran times, running them in the future comes much easier.
I would recommend now that you're out of high school that you don't race anywhere near as much, just keep consistent mileage for 6 months or so and you will very quickly be back in shape.

I'd also look into some sport psychologist books. You'll have hurdles along the way, got burned out 1x, starting back up , etc. Having intermediate goals, plans to deal with success (not getting carried away) and setback (coming back too hard too fast), etc. It'll be hard work, just remember to enjoy the process too.

Previous posters points about getting carried away and too much too fast are very important. What you did in the past isnt currently relevant and what yoy might do in the future is only enabled by training within yourself right now. Its easy to get discouraged by a lack/loss of fitness, it is equally easy to start hammering yourself once you realize you’re getting some back. Be judicious about it and you will reap the rewards of that moderation for many years.

You have absolutely nothing to worry about. With some consistent training you will easily break 16 in a year or a little more, but you’re so young, you have like 10 more years of prime running. You could go for sub 15 and, heck, even sub 14 if you truly dedicate yourself and you want it. I may be going overboard since you just said your goal is sub 16 but Im just letting you know what’s possible. Good luck

I’ve been exactly there due to injury.

Get ready for 3 mile runs at 7:30-8:00 pace feeling like a tempo run, but it’ll come back quick if you haven’t gained a ton of weight.

Just make sure to get out the door and do something 5-7 days a week, and a couple months from now you’ll have that realization that you are starting to feel like your old self. It’s a good feeling.

Thank you. Do you have any books in mind that you recommend?