The short answer is, you can’t really break in mountaineering boots. They’re engineered to have a stiff sole that will hold a crampon, so there’s not much flex. The sole is a “rocker” sole, which will make walking in them a little more comfortable.
That said, it’s a good idea to spend some time in your boots before your climb, getting used to the feel and the weight, and making sure they fit properly so you won’t be suffering from blisters all the way down from the summit of Rainier or Hood.
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