I add my rucking in as a sixth day to my training program, in addition to simply hiking during better weather months 2-4 times a week with light weight (20-30 ish lb hikes). 6th day rucks start with 50 ish lbs and add up to 90-100 ish pounds.
As I add weight, mileage shortens. 60-70 lbs is where I like to peak mileage. I try to always ruck on grade. For me rucking flatland typically wasn't as beneficial as grade.
Weekly fitness is a mix of strength training 3x/wk, LISS 4x/wk, dedicated mobility 1x/wk, long endurance (3ish hours) 1x/wk of mixed modality (individual or combo of bike, running, rowing depending on the wk).
This has been beneficial for me and i continue to see improvement.
BTW, part of my program being close to 50 is minding injury potential, i.e., not rucking in ice, extreme mud, wet scree, etc.
Yes, I tackle those in season just fine, but the pace is slowed and preventing injury in those situations is the focus, not improving fitness.
One more note, as I age, the better dialed my nutrition and sleep, the better the quality of my training and fitness. Sleep is currently the biggest challenge, but if i can hit 7-7.5 hours, I feel extremely good and perform well. Nutrition is primarily sufficient protein with enough good carbs, eliminating ultra processed food, and plenty of hydration (goal is 1 gallon H2O a day). It isn't a secret or magic, but when I hit all these objectives (training, sleep, nutrition) in the proper moderated quantities (not too extreme either direction), I feel blessed and compared with where my fitness used to be a a younger man, it verges on miraculous.