Start off with about 25-35 lbs (total ruck weight on scale). Don't go over 4-5 miles, DO NOT RUN. Take big steps, keep your head up, swing those arms.
This is all you need to train with. Anything heavier is unnecessary wear and tear on your body. When the time comes down to rucking with 80-120 lbs and going 10-15 miles you just suck it up and do it. But training like that will ensure a much shorter career.
Also I'd suggest finding some hills or something with a little vertical challenge to it and remember to adjust your straps when accending and decending, how you wear your pack will determine how long you'll last, dont just crank down on them shoulder straps, alternate between waist and shoulder to prevent fatigue.