I always think when people say they can't run d/t getting out of breath, stitch in side, etc. it is because you are running too fast for your level. True, start out walking w/a few "jogs" then build to walk/run and keep pace slow...conversational (ok, w/ running it is never that conversational). It is a progression. Speed comes once you get a bit more endurance.
To me it is swimming that will be the end of me!