EGF - This will take some time. Everything you do in the meantime should be geared toward making you better and stronger later, instead of trying to push hard and get back to where you were right away.
Forget sub-5 for the moment. In a couple of years, you'll probably be much faster so it doesn't matter. Right now, what can you do to put yourself in a better position to train harder later?
Maybe work on your core. Most young runners overlook this.
Maybe work on exercises to strengthen your feet and ankles, so that running at higher intensities/volumes in the future is more doable.
Maybe do sets of lunges, calf raises, and running drills to strengthen your legs all-around.
Maybe tons of slow, easy running and strides would be an excellent foundation for future harder training. Possible include hill work at moderate effort.
Maybe plenty of cross training. I have used recumbent stationary bike and pool running effectively in the past.
Don't worry. Your fitness will return, and if you do it right you'll be stronger, fitter, and faster than ever before.