We do a good bit of hiking in the mountains in GA and WV, sometimes 10+ miles a day with our dogs and the younger ones both wear backpacks. A healthy dog that's gotten accustomed slowly to backpacking can safely carry about 1/3 of their body weight. Our guys are a 75lb Boxer, a 75lb GSD-Lab mix, and a 70lb Dobe. The GSD & the Dobe do the packing for everybody because they're younger, but Riley has backpacked and does it well.
We use Ruff-Wear. I have a Palisades pack and 2 harnesses. I put both harnesses on my Dobe and GSD mix, and then trade off the bags every couple of hours. Ruff Wear's stuff is top notch for a reasonable price. We looked at quite a few brands and construction techniques before deciding on Ruff Wear. So far, both pack systems have held up well for us.
Balancing the weight is the tricky bit. If that's gonna be a problem for us, I take the Palisades pack because of the water bladders. I can use them to keep the weight distribution pretty even all day. The Approach II packs don't have bladders so you've got to constantly shift the weight to keep it balanced.
Cabelas, REI, and The Sports Authority all handle Ruff Wear stuff and you can get them online for less than in a B&M. A word of advice would be to start slowly, maybe even with an empty pack, for a while. It's not so much getting your dog used to wearing the pack as it is getting her/him used to carrying the extra weight. You need to let them build up gradually to that.
Kidwholaughs is right about making sure that your dog is physically mature enough to handle any serious weight. And remember, water is heavy. Most of the packs I've seen have enough adjustment to allow for some growth, but I'd still want my dogs to be 12-16 months old before I put any weight on them, and prolly 18-24 months in your case before I bought an expensive rig. Your girl's most likely gonna keep filling out for some time yet. If you want to spring for a smaller inexpensive pack just to get your lady used to wearing one, there are some available for a lower price than Ruff Wear. Just realize that it's basically a throw-away, like a puppy collar.