When running a marathon (26.2 miles) it is important to train by doing a weekly run of 20 miles along with shorter mid week runs. It is equally important not to run over the distance, i.e. not to do a training run in excess of the marathon distance.
However, training for a multi day walk, it is beneficial to build up your walking endurance so that you are walking over your projected daily mileage, once a week, for at least a couple of months beforehand. If your planned daily mileage is say, 16 miles a day, then you need to get the point where you can walk 20 miles once a week for the two months prior to your PW attempt.
Dave.