Lots of hand walking to start off with. That is one of the best things you can to do for your dog. Then you can add , spring pole, flirt pole, fetch, swimming, light jogging. Biking or skate boarding. Just about anything you want to do with your do.
Some people prefer not to condition that young, but it is up to you. If you want to condition your dog to a lean and muscular state, work the mess out of her and vary what you do. Road-work, treadmill, springpole, flirtpole, fetch, drag sled, etc. Work her hard and feed her a high-quality food. This is, of course, assuming your dog is healthy and parasite-free.