Sure, walking can make a difference. Any type of activity can and will make a difference.
But you also can't cross that threshold where your body can't take anymore. I firmly believe (imho) that doing more then an hours worth of training a day is self-sabotage. I've been there, I've done that.
I use to do hours and hours of cardio. It got me no where but injured and lanky-looking. Once I did a balance of weight training and cardio and kept it under an hour a day, my results (muscle gain and fat loss) were incredible.
I realize you don't want muscles, right? You are trying to get lean and train for a marathon. You may want to check out other running groups that have trained for marathon's and see how they did it. I have no expertise in that area.
Good luck to you! Marathon's are hard work.