I know a couple of runners who are training for marathons, who are using low mileage training plans. Their mileage never gets above 32 miles a week, several weeks it's dropped to below 15 miles, and some of the runs are less than 4 miles. They only plan to do 3 runs above 17 miles, and a lot of the runs are above or at MP. They both want to break 4 hours but I fear they will be ok for first 20 miles but struggle for the last 6 miles. Neither do any sort of cross training.