How can I best prepare for ruck marching?

Absolutely put weights in a backpack. What is so stupid about that? Then go on a long, long march. And by March I mean a lightly brisk walk. No need to break speed walking records here. The best place to do this also is somewhere sandy. The sand is arduous and most bases consist of sand that you will be rucking through. It sucks. Load about 50 lbs in backpack and you should be alright. Standard fighting load is 40ish lbs. Special Forces? Who knows. Also, weight lift . Mix it up with cardio. Get yourself one of those weighted vests to go jogging in. Incorporate Deadlifts, Squats, and Clean and Jerk into your workouts. Compound exercises are key to good muscle.