My advice is not to hammer yourself. I do alot of weighted pack/rucking and do most of it with 50lbs. You only get 1 set of knees and hips. I'll use 75 lbs sparingly and in the 4-6 weeks right before season.
If you can't drag a pack around, I've found weighted stepups to be a good substitute. Do it HICT style - 65lbs, 10-12 per min on a 12-14" step, keep heart rate below 150 bpm. I use my stairs going to my bonus room and step up on to the second step. Another good alternative for flat ground is a tire drag. I added a shelf to mine and load 10-30 lbs and take it for a walk around a parking lot at my church. I do get some strange looks. I also do trail running with elevation gain/drop.
The other thing I'd consider is running intervals. Run 4 mins hard, walk 4 mins, repeat 3-5 times. Be forewarned it sucks. But so do the 65 lb stepups. I keep my heart rate below 150 and preferably in the 130s. I adjust my exercise speed accordingly - except the running intervals - you want your heart rate near max.
I buy into aerobically training your type II muscle fibers with the stepups, tire drags, running. Rucking for me is more like aerobics for backpacking - it's hits the primary muscles and increases the aerobic capacity of the muscles and cardio system.
Good luck and keep at it!
