I used a Hal Higdon beginner plan to complete my first marathon a little over two years ago. I didn't like the run/walk idea so I never paid a lot attention to the Galloway plans. I have a running buddy who used a Galloway plan to run her first marathon and it worked just fine for her. Don't think you've failed if you have to do some walking in order to complete your first marathon.
The length of your longest long run is the subject of much debate in the running world. I don't know first hand if the interval method really helps your recovery. I would think that dialing down your pace would be just as effective.
One of the most important things you will learn as you train for your first marathon is how to manage your rest/recovery. Don't feel like you are locked in to the patterns of rest that are in these published plans. You may find that you want to shift your rest days around a bit. One thing I would strongly encourage you to avoid is hard workouts on back-to-back days.