What should a 15 year old do to improve aerobic capacity?

Eat kale and do lots of pushups.

I suggest to keep it simple. Do easy distance. Work your way into long runs. Target 90 minutes or more. Don't just suddenly double up your usual 45 minute runs; ramp up.

seeing XC is in so long just slowly build up mileage doing mostly easy runs, like EASY. Then throw in strides for turn over. Include a tempo run, not getting complicated early on do like 20 minutes @ a pace about 50+~5 seconds over your two mile. Don’t use this to guess your shape, it’s meant to build it.

every week you can do some kind of faster workout. Remember, fast but not hard. You could do something like a fartlek with 15 seconds surges every landmark you see, or have 2 minute jog recovery, or do a hill fartlek or sprints, like maybe 2(4x15s@mile) with 2-3 minutes rest. This is meant to maintain speed and not push your limits. You can make all kinds of things to mix it up but you just want to keep it fast and not hard.

the focus isn’t on guessing shape but focusing on improving aerobic conditioning which is only achieved through recovering and a lighter load of this type of stuff.
if you are confused go to one of the million summer of malmo threads

At 15 I wouldn't recommend you do any type of serious "training" unless you are a prodigious talent and have a world class coach who knows what he is doing. Injury at your age could have long term repercussions.