I've run an half-marathon once, a few years ago. I play futsal (it's similar to indoor soccer) once a week, and I do aerobic exercise such as biking and running, and weight training. I recommend everyone with CF to exercise, especially aerobic exercise, because it strengthens lung function and the immune system. For those that can't stand 5 minutes running don't worry, that's normal in the beggining. I started running for 5 minutes in the beggining my preparation for the half-marathon, but one, two weeks later I could run for 20 minutes, and then half an hour. One month and a half later, I could run for 2 consecutive hours.