How much should I run? 13 y/o girl?

I feel like I need to run and I'm pushing myself to do it cause I'm lazy, but I haven't been able to start yet cause of the weather. I have mild chronic asthma and I have since I was 4. I don't get alot of asthma attacks, and I can breath pretty well, I just have alot of issues breathing while running long distances. My doc said the more I do it, the more amune I would be. So how far/long (miles/time) do you think I could run? Also exercise tips to do at home would be helpful! Thanks!