I've read the book, but haven't tried the program, because I'd rather run than cross-train.
It's basically a program with 1 long run, 1 tempo run, and 1 interval session a week plus 2 or 3 cross-training sessions. Despite some of the other comments, I don't think it's actually less effort or time than most "1st marathon" plans. It just replaces a lot of the miles with biking, swimming etc.
I agree with the comment above that if you can hit the times and run the distances in the program then you are already in decent shape for 50, so I expect it would be fine using the system to get through a marathon. The risk of injury is probably lower than an all-running high-mileage training plan. It would probably be a good plan for someone who also wants to be in shape for triathlons. It's not the optimal program to run the fastest marathon you are capable of running, but that's probably not your intention for your 1st marathon anyway.