With a young dog, before the growth plates close and the bone ends in the joints harden, should not be formally conditioned. A light load, like half a pound on a 50 pound dog is fine. I let my adolescents pack their own training gear (extra collars and leashes, bandanna, training treats, water bowl--I carry the water). I have my adolescents pull a 3 pound red brick with a harness. Just enough they can feel the bump and jerk against the harness, but not enough to stress them physically.
You also need to be careful about the surface she is working on. You can also damage joints and bones working a young dog on hard surfaces, such as paved surfaces. It would be best to minimize that and try to do the bulk of exercise on turf. I realize that may not be possible for everyone, but it is the ideal and you need to understand the risks associated with working on pavement. My pups do ordinary walks on sidewalks, but get most of their exercise on turf or jogging trails. If it won't give a jogger shin splints, then it's good for a puppy too. Also don't over do any physical activity. Young dogs tend not to have the good sense to realize they are tired or sore and stop or rest. Snort, it took Cole 9 years to start voluntarily resting during play without being told to do so. You have to decide for them. As with training, short, frequent sessions are better than less frequent, longer sessions. As with nutrition, a wide variety of safe physical activities is best. As many terrains as possible, as many different (comfortable) motions, including stepping sideways, backing up, turning, and generally learning where one's backside is at all times. If you're going to do agility with a pup, make sure you spot him well (stay next to him on raised equipment and steady him if he mis-steps before he can fall). But with spotting, agility obstacles can be great confidence boosters and practice in dealing with different situations.