Your training plan sounds fine to me. It may or may not be perfect, but it's adequate. Just as important as the physical training are the things you'll learn as you increase the distance. Like how to manage your fluid and food intake to carry you past 150 miles, past 200 miles, all the way through the finish. And how to get through the rough spots when they come, and how to deal with little issues like chafing or hotfeet before they become big issues.