Often times runners encounter injuries from overuse, bad form or an accident. Usually we seek the advice of a Dr who is trained to treat our ailments. Doctors are not running coaches and not very many of them know enough about running mechanics to offer anything more than, "don't run" or "no long distance running for you."
While I do not advise against seeking the advice, diagnoses and treatment from a doctor if you are hurt/injured, I highly advise seeking the advice of a running coach if you are injury prone.
The key to a long successful life of running, walking or run/walking is making sure we stay healthy, which is the reason most of us are out there.
Once the doctor diagnosesyour problem and offers his treatment, I would make sure you fully understand what they are diagnosing and what part of the body they believe to be injured. Then, with that information and the experience and knowledge of a running coach, you might be able to diagnose the problem. Your knee pain may actually be coming from many other things not in your knee.
As far as walking the half marathon, yes, you should be able to do so depending on the races cut off time. The Disney Half Marathon has a very generous cut off time and you can make it while walking every step if completion is your goal. As others have suggested, make sure you check the cut off times prior to registering to make sure your 18 min/mile pace will come in under the race cut off.