The trail 25 has no supportive belt and a very light frame so probably is designed to carry up to about 10-15lbs of stuff...generally the max amount that is comfortable directly on the shoulders and compressing your back. ymmv. You can carry more for the purpose of exercise as it seems you do but you will probably find it uncomfortable for any extended period and personally I wouldn't. Weight shirts are probably a better approach to this kind of thing because they distribute the weight evenly around your body...but not my thing so just speculation on my part.
50lb is a lot of weight to carry in a backpack and generally only backpacks of capacities of >50L are designed to carry that sort of weight. They generally have a stay or some kind of fairly substantial inner frame and a supportive padded hip belt that transfers most of the weight to your hips. Even with that capacity many of them run out of comfort at around the 35lb mark. Of course that is for hiking all day backpacking rather than for weight carrying exercise.