As someone who has coached hundreds of people to walk marathons, I would say that one of the most important things is that you have to get used to being on your feet for hours. While speed work and everything is important, you have to be used to being perpendicular to the ground for hours on end. Make sure that your training program has a least one long walk a week, and a few medium walks.
Also, if you can watch your form, and get anyone to give you some pointers one having a smoother, more efficient walk, that will pay dividends down the road. When I was more actively coaching, I would spend the first 6 weeks on technique, and not worry about distance for a while. If you can see if there's anything that is jolting and not smooth, that's something you should try to fix.