You'll need to do plenty of training if you're going to walk 38 miles in one go. I'm more used to multi day walks rather than one long one, though I'm sure a lot of the same principles apply. If you don't do any walking at all, I would start now by doing shorter walks of say 5 or 6 miles and then build up. If you need to break boots/shoes in, wear those. Wear whatever rucksack you will be using, packed with the weight so you are used to it. Others will be able to advise better on distance you need to be able to do, but I would have thought you would need to do several 20 mile plus walks before you tackle 38 miles, but not too much in the week before.