For aerobic endurance conditioning I had to find something other than running. While I can sprint fine as needed, I just plain hate running for long time & distances, shreds my 63 yo old shins and ankles. And, also wanted any endurance training to be as close to realistic rucking when under load, too, so…
Every third day I do an hour and half on treadmill set at 10% incline and brisk walk it in my combat/hiking boots at (currently) close to 3 mph with a 38 pound weight belt. Year ago started at flat and when I got up to 3.5 mph, raised incline to 5%, then when back up to 3.5 mph again went to current 10% incline. I’ll raise it yet again when pace built back up to that 3.5 mph.
I try and keep my HR under 120 bpm, which is easy conversational pace, still firmly in the aerobic fat burning range, which is much easier to maintain for very long periods. I’ve watched my pace steadily increase at that same HR over the months, which clearly confirms aerobic conditioning is effectively improving. (Maffetone heart rate training is what this is, for any interested.)
I’ve also seen my weight training, done following day after treadmill workout, benefit from this improved endurance & stamina, too, with my being able to handle ever more weight for longer sessions and then later on even recovering faster, too.
Most of my real world pack weight is set up to transfer load directly to my hips, so this training should be pretty spot on in the field with utilizing the weight belt and wearing the boots, specificity wise.