I am just basing my views on this subject on research done with members of the military who ruck very heavy loads for long distances. The best results were from 2x weekly running, 2x weekly complete body strength training, 1x weekly endurance/intensity rucks and 1 day a week of metcon which could be pushing/pulling a sled or something else.. To me, if you have leg joint issues, endurance/intensity swimming is something that can replace endurance/intensity running. That program recommends working up to 70# for longer endurance rucks and 90#+ for shorter intensity rucks. Swimming can be a very good cardio exercise depending on how you structure your sessions and if you have leg joint issues it is a very good option, imo.