First, I think that the traditional all leather hiking boot is out. People that start with this type of a boot seem to loose them real fast. They are heavy.
Most hikers use a leather and fabric top mid height boot, or trail runners like the New Balance 805 series.
Now, these boots don’t really need much breaking in. But, what is needed is for you to condition your feet to the shoe (I wouldn’t share my story now).
Feet do grow (spread) because of the additional weight carried, plus being on your feet for so many days repeatedly. About the only way to minimize it is with a good footbed like superfeet. But, I definately wouldn’t advise haveing a second pair of footwear ready to go when the first pair gives out. For one thing, they probably will not fit, and for another, your feet should be so conditioned by that time that anything that fits reasonalby well shouldn’t give you any problems.
How many miles you get out of your footwear depends in part on how hard you are on them. Usually, people get 1000 miles or so out of a pair. But trail shoes are built much lighter. Some people only get a few hundred miles out of them.
Peaks