I've recently started running again after a five month break to recover from a shattered knee cap. While my legs are strong and my knee feels okay, I've had the hardest time breathing when I run outside. I can knock out a five-mile run on the treadmill with no problems, but whenever I street run I suddenly feel asthmatic after only a few blocks and must stop and walk a bit. Once I walk, my breathing stabilizes quickly, but when I begin to run again I almost immediately start to wheeze. Does anybody have an idea as to why this may be happening and what I can do to get over it? I'm training for a half-marathon in April and want to street run as much as possible. Thanks!