Gary,
All depends where you are now really. For instance, to run sub 3 hour marathon, how long do you think you'd need (training wise) to run 18 miles in a time of 2:03 fairly comfortably? Because that's the pace you'd need to keep running at, 6:51 min miles for 3 hours.
Have you look at the results of the races for previous years, to get some idea which ones might be a faster course? You might be able to find maps with incline information. MapMyRun has some maps of courses showing incline, or the event websites might have them.
Paul