I guess I do to train for elk season, although I have never referred to it as rucking. I do 2-3 miles, 2-5 times/week starting around May/June, with a 45 pound sandbag + 2L water in my MR pack. Weekends throughout the year I'll mix in much longer trips. Once every couple weeks I add a second sandbag to get to 90 lbs, but that kind of weight isn't good for the joints on a regular basis. I have a couple 1,000' ridges nearby that are really steep so it's no doubt a really functional hunting workout. If you live in the midwest and only have flatlands nearby, I would imagine you still want to be building the climbing muscles on stairs or whatever you can.
I don't do distance running so I struggle to keep cardio interesting as well, especially in the winter. I mix in mountain biking and kayaking to keep from getting bored throughout the warmer months. Once a week i run intervals/wind sprints - sprint 80-100 yards, jog/walk back, go again right away. Repeat 8-12 times depending how I'm feeling. In my unscientific opinion this builds the lung capacity faster than anything else I do.