Congrats on the completion of a very long march! The longest I ever did was 15 miles way back in the days of my youth.

Here's the tricks I remember. Take a sizable dose of Ibuprofin right before the march. Did you use snug socks and well worn but snug fitting boots? Did you put those big civilian sole cushions in your boots? Did you change your socks regularly (every one to two hours) and apply fresh powder? Dry powdered feet are key to avoiding blisters! If you don't have enough socks, you can leave your other pairs hanging off the ruck to dry and rotate them. Training up some calluses on the feet prior to the march is recommended as well.