When you buy or get issued a pair of boots you have to break them in before you run / ruck for miles in them. Over the years I have tried many methods. Here is one method that worked for me for at least ten pairs of combat boots.
Get them soaked and walk in them for a day. Soak your feet and boots in some form of water (ocean, lake, river, bathtub, even shower with them). Just get them soaked. Then take them off after about 30 minutes of walking in them and replace your wet socks with dry socks so your feet do not get too soft and start to fall apart. See my video here: https://www.youtube.com/watch?v=qvPMTI5gO0c
Take a pair of thin socks under your thick pair of socks. I personally like a polyester / rayon blend that cling to your feet / lower legs like a pair of stockings. If you ever played baseball and used baseball socks you will know the kind I am talking about. A better picture maybe your Grandfather wearing a pair of shorts with his dress socks and shoes — that is the sock I am talking about! Then put on the outer sock. This is whatever you are issued — Army Green or Navy Dive socks for instance.
Continue walking and picking up the pace to a slow trot occassionally to start to breaking them in the way you walk, run, and ruck. If you can keep a pair strictly for rucking and one for running you will see a difference in the way they feel as you ruck differently than you run. This is not necessary but it could save you from some overuse injuries over time.
Make sure you do a freshwater rinse and polish them with leather softener / shoe polish for extra protection and comfort.
Take care of your boots and they will take care of you!
Military.com veteran community — share some of your methods as there is more than one way to get your boots ready for action. Comment below.