Although, I'm not familiar with Giant dogs quite yet. I do know that with large and medium dogs (under 2 years of age) you don't want to "work" them hard. The less strain and pressure on the bones, the better I suppose. The same, probably applies especially to giant breeds. I don't think walking them "too much" is going to do anything. It's more straineous, extensive physical activitiy that can possibly damage the growing bones. You could probably walk your Saint all you want and she should be just fine, as long as you keep a steady pace. If you notice your dog is getting tired then perhaps it is time to relax.
I really wouldn't worry too much about her hips or bones. I mean, as long as you aren't making her jump over things, climb a flight of stairs, making her weave in and out of poles that are placed together as tightly as possible...you r dog will be fine. I think that simple excerises are best.
If you would like to get your dog into some kind of sport, you can. Being a puppy and depending upon the sport, she probably couldn't do anything until the age of 2...but she can most certainly learn the routine.
You can even try to be creative and invent some new games. Perhaps do the "find it" game on a much larger scale. If you have 2 acres, that should be easy for you. Instead of treats, try using people that she loves! Take her senting skills to the next level.
You and a few others from your family can get into a circle. Then each take a turn on calling her to them.
Chasing games are quite fun too, so long as no one gets tackled by the dog! lol.
Does your dog like people and other dogs? Perhaps you can invite some people over or bring her to a friends house. Take her to some classes where she can meet and interact with other dogs, perhaps even dogs close enough to her size. Take her to the Beach, the Park or someplace that's new that may spark her interest.
Just some ideas...
...good luck on your adventure to new games!
=(^ - ^)= Silverstorm