FIRST Marathon Training

So I'm thinking of trying out the FIRST plan for my first marathon.  No big goals, I'm planning to do a moving bday party with a friend and she's estimating about a 6 hr finish time (to allow more people to participate).  I do however, want to know that I can do that distance without dying.  6 hours is would be slightly longer than 2x my half marathon times.

I know the concept of the plan is that every workout is to be like balls to the wall since you're only running 3x wk.  But does that plan work ok with a run/walk?  If I had to, I could calculate my run/walk interval to equate to the pace I'm supposed to be holding, or I can just do the run @ desired pace.  Just curious on people's thoughts.