Try On Your Shoes Carefully Before Buying Them, Or At The Very Least Before You Go Walking
Whether in a store or at home when receiving an internet order, make sure you try out your shoes.
Put them to the test in a variety of situations: on the flat, climbing, descending (stores often have slopes or mini-routes which simulate different types of terrain).
If ordering online, give yourself a safety margin in terms of a potential time period for exchanging them. Do not get them delivered at the last minute to your holiday destination!