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Marathon training does require planning, but it's important to remember that training schedules aren't written in stone, and you won't jeopardize your race if you can't get in a 20-mile run on the weekend you planned due to excessive bad weather, family emergencies, or illness/injury.
Just look ahead, and do that 20-mile the following weekend. Or if you know that you are going out of town for Memorial Day, for example, do your longer run the week before, and take an easier weekd of shorter runs when you're away from home and probably won't know your exact mileage. The important thing about marathon training is to be consistent, and get in the mileage over the weeks and months leading up to the race. Do that, and it doesn't matter whether you did that long run on this Saturday or that Sunday. Or even on a Monday, if that works for you.