Research: read up a bit on exercise-induced asthma from the Mayo Clinic and Web MD
I know more or less what the symptoms of asthma *are*, but I'm not sure of some of the details on when they'd hit, how/when my character would treat them, and so on.
Our Hero is slightly overweight, not in particularly good physical shape (think typical nerd), and has asthma (at least partly exercise-induced), but is in generally reasonable physical health otherwise. He's in his mid to late teens (probably 17 or 18).
He is being chased by demons, along with a group of other teens (including his boyfriend). He needs to run for his life, basically for most of the duration of a typical feature film (though with short breaks), through a swamp-like area (maybe not quite a swamp, but there are lots of creeks and streams). Worst case scenario, his boyfriend (a big, very fit jock) can pick him up and carry him, but that is obviously not ideal, since the entire group is running. Said boyfriend can easily help him along a bit, however.
So. Would he likely start having trouble breathing during the first 5-10 minute sprint, or not until after they stop to rest/some later point? If his asthma starts acting up while he's running, would he be likely to pull out his inhaler and use it on the go, or would he need to wait 'til he's stopped? If he has to stop, (how well) would he be able to keep running until the entire group stops? Would he need to keep using it, or would once be enough? If he'd need to keep using it, how often? How, if at all, would massive doses of adrenaline (from, ya know, sheer terror) affect things? Anything else I need to know?