I just did the Runway 5k this morning at the Charlotte Douglas Airport.  The course is really fun - right on the runway and planes are taking off.  It is flat so it is fairly easy but some of the course is on concrete so my joints were hurting a little.  The race was VERY disorganized because they got about 600 more people than they expected and the start was pushed back by 30 minutes. The traffic going in was very frustrating and once you parked, there was not a clear route to the registration tables.  Once it got started, it was a lot of fun.  I would do it again next year but hopefully they will be better prepared.