I have asthma and I'm generally exercise induced. Kind of counter productive to backpacking.
I have good days and i have bad days. On good days i can do 20+ miles, bad days i'm lucky if i can cover 3 in a day. Problem is even on a good day i can make it turn into a bad day, if i push too hard my lungs go on strike. The warning sighs are not always obvious. I have literally hiked until passing out. I don't always wheeze either. I've learned to always pack enough gear be some what comfortable if i am forced to stop and say overnight. What i pack depends on the season.