I tend to spend a lot of time with a backpack on year around. In the winter and Spring months, I do a lot of backcountry skiing wearing a backpack with an airbag and avy gear.
April-Nov, I do a Wednesday morning hike that is unweighted (no backpack). 1,000 feet in 1.2 miles for time. In the winter, this is either skinning or a gym conditioning workout consisting of rowing, box step ups and KB swings.
And then the weekends through the Spring and Summer are either out turkey hunting, day hiking, backpacking (I do quite a bit of this), backcountry fishing (usually backpacking), peak bagging etc. If I'm limited on time, I'll still usually get a training hike of some type in. I also do several extended backpacking trips consisting of around 5 days or more each summer.
I personally don't ever do heavy, intentionally weighed rucks as I don't find it necessary. While you need to have your body specifically conditioned to wearing a pack, moving efficiently and effectively with that weight and fully realized muscular endurance fitness associated with rucking, when you get into packout weights, I find that is where the strength training takes over. In other words, if you have strong body from putting your time in on the compound barbell lifts, and you have developed your muscular endurance under some kind of typical pack load, say, 35-60 lbs, then you do not need to spend time hauling around 90 lbs, 100lbs, 120 lbs etc for training purposes: you'll be able to effectively handle and haul that weight due to your developed strength and muscular endurance.
Relying on backpacking alone to improve your strength is probably the least efficient way to develop the necessary strength.