In planning for the worst you are stating what you can see as the last ditch, "this is the only way I'm getting where I want to go" plan. For Dave, he can't see that he can get into this park any other way than to accept a seasonal position. And in doing that, he is not even guaranteed a regular part-time position.
There is a faith factor in Intentional Winning In Life. You have to trust in the beauty of your dreams and even if you can't see it, know your dream is coming true. Call it faith, intention, or the Law of Attraction - no matter what you call it, when you focus on something it grows.
If Dave does some of the little things related to his Gettysburg dream, who knows what will happen, what opportunities will present themselves? Who will come into his life and show him other Orange Roads? These are unknowns that instead of being scary, can be exciting! Get excited about what's to come while planning for the worst - Dave can enjoy this time with his son, dig into all the little ways he can live his dream, enjoy planning his tenth wedding anniversary in New England, finish school, and on and on. There's so much more to this life, right now than what he can ever plan for his future.