I did the Galloway program last year when I did my first marathon. I did 4/1 run/walk ratio in training and then a 5/1 run/walk during the marathon. Don't feel bad about being slow on 18, it's just a training run. I remember many bad days during training. But a walk every 10 minutes doesn't give you the amount of rest that the Galloway program is designed for. I'm now training for my second marathon (Stockholm), and I have dropped the galloway program, but I give it credit for getting me through last summer. Also, on your first race day, don't worry about your time. Just enjoy it!