Apologies for long post in advance.
For a high school runner with limited leg speed hoping to run a sub 4:30 mile, I would recommend the following workout to build up tolerance to race pace.
3 sets of 3x400 in 67 secs with 45 secs rest followed by 1200 in 3:50 with 5 mins rest.
If you're all by yourself, you can cut the long interval down to 800 in 2:30 and the set rest down to 3 or 4 mins. Whatever your long interval, keep the rest short between the 400s and your set breaks no longer than 5 minutes. If you can hit all nine 400s in goal mile pace you will be able to maintain pace from 600m-1400m where most people lose concentration and rhythm.
It may be necessary to start off with only two work sets but with your 9:30 2 mile I am assuming you can handle the volume. It is important to hit consistent 400 meter times AND cover the long interval at pace. The long interval doesn't need to be track fast but it needs to be sub tempo pace to maximize aerobic training effect and keep the 400s honest. If the 400s are too easy, (they won't be if you keep the rest short) you can always run the last set faster.
IMO, this workout has three benefits not necessarily offered by speed intervals or partial distance time trials.
1. The short rest 400s force you to run evenly at mile race pace rather than surging through the start or finish of each interval. (This ability to maintain an even but fast pace is particularly important for someone with limited leg speed who cannot handle dramatic pace changes)
2. The moderately paced mile interval forces you to rely on your aerobic system after you’ve already built up some lactic acid. However, unless you bonk, the long interval actually gets easier as you clear the lactic acid from the 400s out of your system. This is a big confidence booster and with a decent set break your body has a chance to reset before the 400s start again.
3. The last set of 400s is a bear and requires concentration. Although it is not a race-simulation type workout, this set does teach your body to run fast when you are experiencing both lactic acid burn and overall fatigue. The rest is still short so you don’t have time to whine or psyche yourself out. After your last 400, then it is time to stride out one last long interval.
In hs, I ran too short of intervals too fast with too much recovery. I never really learned how to relax at race pace. Not surprisingly, after a growth spurt and performance jump between soph and junior year, my times stagnated senior year and I wound up with very marginal hs prs of 54, 2:00, 4:30, and 9:39.
I walked onto a college program that ran moderate base miles and decent amt of mixed intervals. By the end of indoor track, I dropped my mile time from 4:30 to 4:16 while training primarily for 5k.
The workout above is based on an indoor workout where the 5k group ran:
3 sets of 3x400 in 63 secs with 30 secs rest followed by a 5:00 mile after 30 secs rest. 5 mins rest between sets after the mile.
A week later during a tune-up race, I felt like the workout helped me pull a decent third lap even though I did not feel that great en route to a new mile pr.
In terms of speed, I would work on coordination and agility drills over the winter and through the season. However, be sure to keep your base up as I imagine that is how you got to a 9:30 2 mile in the first place.
Good luck and keep at it. For now, lowering your mile time should just be part of improving your overall performance.