If you want to see serious improvements in your fitness level, you have to push your body to the limit – then do it again. The processes that makes your muscles feel sore also strengthens them and being winded trains your body to better conserve oxygen. Though pushing your physical boundaries is healthly, pushing them too far can be dangerous and debilitating. The key is to know when you're overdoing it.
Dr. Christopher Kaeding, professor of orthopedics at Ohio State University and head team physician for the Buckeyes offers some insight.