Even old, comfortable boots will harden after a few months in the cupboard, and need to be broken in again before the trip. There is no substitute for just walking in them, although soaking in water beforehand can help.
If you feel a blister coming on, stop immediately and apply plaster (rather than a bandaid). Once you have blisters, there are special remedies such as “Second Skin” that you can use to cushion the pain.
When buying footwear, it is better to err on the big side, and wear extra socks, rather than get something too snug.