Many thanks to you @OldGuyot and @Wanderer .  I am kinda old (51) so trying to be aware of my physical limitations, while also surprised at how easy and fun rucking is.  I will abandon the idea of adding more weight to the Trail 25, and look to transition to some other thing for my workouts like a weight vest.  It's such a nice bag IMO, I'd like to make it last as long as possible.  Thanks again!