Until I hurt my back in February, I also regularly carried a weighted pack for training.

Since I carry it back and forth on my work commute, I don't want to use a large pack. So I train with an old alpine type climbing pack that still has padded shoulder and hip belts. Since it's relatively small I load it up with dense stuff including a battery and bricks. Last year I often carried 35 pounds which might be even more than what I'd have for a backpacking or overnight climbing trip.

Like Brian, I don't worry about using a different backpack or having the weight distributed differently between training and hiking. I think it still gets muscles and tendons and such conditioned for whatever pack you use while hiking. I do think it's important to get muscles and joints used to carrying weight downhill, so once a week I did a stair workout with the ballasted pack.

For what it's worth, I've taken a couple of long hikes with partners about half my age and felt good during and after the hikes. https://landmarkimages.smugmug.com/Other/Outdoor-Adventures/2015-Highlights/n-CWjxCF/

It so happens that today is my first time back carrying weight (not 35 pounds ; )

Have Fun,
-Bob