No, you don't have to train at 26 miles before you race your first marathon, but it sure helps. I ran 26.22 miles four times before I entered my first marathon. Two of those runs were on the actual marathon course. It sure was nice to know exactly what to expect on race day.
If a marathoner hits the wall at trail camp, it's probably the altitude, not lack of conditioning.
I agree that getting out and hiking is the best training for hiking. Early in my climbing days, I climbed Mt. Raineer with RMI. At orientation, the guides asked everyone to introduce themselves and tell everyone how they prepared for the climb. My wife and I were the only ones who prepared by climbing.
