OK, i used to have asthma and go figure i wanted to play soccer in H.S. and i wasn't gonna let asthma get in my way. But in order to make the team i had to at least be able run 10miles without stopping and my jaw like dropped, so it was time to train.
*You will not be able to do this if u slack* (let me tell u that right now)
Heres what i did:
1st week: i ran to my breaking point. Like everyday i would either lightly jog in the morning or at dusk until i could feel my asthma flaring up. (keep inhaler on u :duh

but yeah, just a light jog until u felt that your couldn't go anymore. Dont worry about distance or time, just worry about trying to balance your breathing, and muscle control. Trust me it sounds weird but there are certain muscles you can not use while running if you just don't use them. (worded weird) but also focus on not thinking about anything else, because it will mess up your breathing. When u get to your limit, do not stop... start walking, and put your hands up above your head. until you finally come down and get back to homeostasis. Drink your water, and make a note of how far you went. Repeat for the rest of the week.
Next week, i payed attention, to actually how far i could go. Like i had my timing down, and got a nice pace from the 1st week, now i kinda started to push myself. i just tried to go further and further. But don't push yourself too hard. One day i did, and my timing was thrown off, and i couldn't focus, and i needed to use my rescue inhaler. it was bad. But yeah, don't push too hard.
THe next weeks after that all i did was find a good distance, like half of where i first stopped the first day and the furthest i went the 2nd week, and i made that my stopping point. Then with this spot in mind, i started to cut my time, like increasing the pace little by little. But for you. You can just try to increase your distance. Remember some days you wont be able to go as far as you were the day before... don't worry about this.
Then from then on i just tried my hardest to get my rhythm down again, and just increase distance, and i did this until like a week before the fitness test/tryouts. Then i didn't run like 2 days before the run, all i did was tank up on water. and the 3rd day before i just ran to my original point (from day 1) and back to my starting point. I made the 10mile run, and i hated every minute of it, but looking back on it i was like "YEA!"
NOTES:
Get a full rest everyday
Eat healthy
Drink lots of water
Do not bend over or stop after running
Dont try to hard
Stretch before u run
If i think of more i will edit this post
Eventually i played soccer so much and ran so much, i built up my lungs. And then like senior year my doctor said i had grown out of it pretty much, because of all the running and stuff, and he took me off my medication.