Hi AnnMarie! Wow! I am impressed that you are considering the Goofy Challenge! I think whether you do the full or do both should be totally up to you. Not knowing what your running level and ability is, I hate to make a suggestion either way. But if you do decide to do it (and I hope to do it at least once!), I can recommend a training program for you. Jeff Galloway is runDisney's official training expert. He has a customized training program for every Disney race. His plan for next year's Goofy challenge isn't released yet, but you can take a look at
Jeff Galloway's Goofy Challenge training plan that was for this year's marathon weekend. It is a 28 week program with the goal of "finishing in the upright position"!! You can adjust the dates to work for next year's race. There will also be training plans out for the full marathon and half marathon for next year.
You will need to arrive sometime before around 5 PM the day before your first race (either the half or the full), so that you can pick up your packet with your race number, t-shirt, etc. Packet pick-up is not available on race day mornings. There is also an awesome Health and Fitness Expo where you pick up your packet that you will probably want to spend some time looking around at. There are vendors with everything imaginable and even a guest speaker series full of great advice.
It sounds like you are going to have a wonderful trip! Please send me any other questions you may have, either about the races or just planning your families days at Disney. Also, let me know if you have any questions about some of the runDisney offerings for families during races (chEAR squad, etc.). Happy Training!!