Please accept my condolences on the loss of your dog. It's never an easy thing.
Dogs and Hiking, two of my favorite subjects. Where to start...
It sounds like you've done a decent amount of homework already, since you're aware of growth plates and possible orthopedic damage. Sardog1's suggestions are excellent. I think it's okay to see a little rib and even the hint of a vertebrae or two.
I would start getting a puppy out into the woods immediately. The trips should be short, full of fun and positive reinforcement. If you know the term "socializing", that's exactly what you want to start doing for anything you expect that Pup will be exposed to throughout her life (not just the woods but also tents, packs, snowshoes, etc). Praise the behaviors you want, ignore the ones you don't (such as praise for picking her own way through a tricky area, but ignore barking at other hikers). Although I've never gotten involved in agility (a timed obstacle course for dogs), some agility classes could be an excellent way to socialize Pup to moving around/through/on obstacles. A good Puppy Kindergarten class would be great, if you can find one. If you know a good hiker-dog, it'd be great if Pup could get out with them for short outings - dogs are good at learning from each other. Again, Sardog1's guidelines about beginning some training with recalls and such are excellent advice.
I use a plain 6' cotton web lead, though I've been thinking of going to a 10' for hiking. I don't like flexis for a few reasons: they break, they do not necessarily limit a dog's area properly, and they make leash training more difficult. Leash training is more difficult because the leash length varies, so Pup has difficulty in knowing what to expect. With a regular lead, they learn the length. I also practice doing "me first" and "you lead", which are cues to the dog that either I'm in front or they can be in front. I also try to get them into the habit of passing only on one side (by picking the side, then not letting them pass on the other side)
As for physical training, I've had dogs on the small side of giant for years. Since Pup is a lab, if you're near water, swimming is a great, low impact exercise to keep her in good physical condition and she can do as much as she likes at any age. Below are the guidelines I try to follow for hiking.
birth-6 months: mild exercise, playing. Avoid jumping down onto hard surfaces. Avoid slippery surfaces. Avoid splaying of the rear legs out to the sides. If Pup is laying frog-style (belly down, rear legs splayed), reposition her. Walks to be no more than 1/2 mile at a slow pace, no more than 4x/day, carry over any terrain that would involve jumping, easy scrambles are okay. Some people will carry the puppy down staircases at this age, especiall if there's not much traction. Begin introducing Pup to any equipment she might see in the future, such as booties, packs, harness straps. She doesn't have to walk in them, but she can get used to the feel of something on her body.
6-months to 1 year: still avoid jumping on hard surfaces and splaying of the rear legs. Walks may be up to 1 mile no more than 4x/day, and may involve medium-easy scrambles and small hops down. Walks may also become brisk.
1-2 years: Walks up to 5 miles allowed, length should vary with difficulty of terrain. Scrambling and easy hops allowed. Pace may become faster, keeping the dog at a trot rather than walk. Empty packs allowed.
2-3 years: Keep walks under 10 miles, challenging terrain allowed. Packs under 5 pounds (<5% body weight). Okay to begin runs under 5 miles.
3-4 years: Under 20 miles, depending on terrain (longer mileage allowed over easier terrain). Packs under 10% body weight. Runs under 10 miles.
>4 years: by now the dog should be completely physically mature. Any major joint/orthopedic problems should already have been detected. Mileage and terrain limited only by dog's capability. Packs <20% of body weight.
If you run daily, dogs should run no more than every other day.
Following this, I've had dogs that hiked every weekend with me, could do +30's, multi-day trips, and carry packs. With that said, it's important to keep them on a regular exercise regimen to keep them in hiking condition. None of my dogs have had (yet, knock on wood) any joint or orthopedic issues such as arthritis, dysplacia of hips or elbows, or osteophytes on the vertebrae.
Transition to a maintenance formula adult food at around 3 months. It'll help Pup grow more slowly which is thought to help prevent orthopedic problems.
I don't know if heart conditions are an issue with Labs, or an issue with the line Pup comes from, but it wouldn't be a bad idea to have the vet run an ECG one every couple of years to keep an eye out for murmurs, irregularties, etc. Some cardiac problems would preclude hiking/exercise.
Some of my favorite dog training books: Karen Pryor: Don't Shoot The Dog, Jean Donaldson: Culture Clash. Ian Dunbar and the Volhards usually have good stuff too. I have a couple of others too, but those are on the shelf at home and I can't find them on Amazon at the moment.
Best of luck!