I started off last summer on a 34-day trip (Roper's High Sierra Route) with a new pack never worn and new shoes, never worn. It worked out just fine. I did put all my gear in my pack at home to be sure it fit, put it on once, and decided it was more comfortable than any of my other packs. To be fair, the shoes were same brand as the old ones that wore out. I do a lot of research before I buy gear so seldom have any surprises.

My concern for the question posted, was that if you are carrying extra body weight to begin with, I would not add more weight of a backpack for "training". Just walk first, or else you can put too much stress on knees until you strengthen them.