I think it is all about building up to a certain distance and pace. I have a 18 month GSD and we run 4-8 miles, twice or three times a week. On "rest" days we might not do anything or go on 2-3 mile walks. On weekends, we might do a 8-10 mile hike. We're very active.
Some might feel this is too much, but I know he enjoys it and he's very capable of doing it. It's about listening to his body, seeing if he's keeping pace and building up to it gradually. When we first started, he would lag and lay down when he was tired, even in the middle of the run. So I knew we had to walk or give him some time to relax. He never does that at this point. In addition, he doesn't get "growing pains." Some dogs have these up to 2 years or only up to 12 months, every dog is different. Once your past that stage, I feel that you can really start good physical training if you want to.
As far as food, I feed 5-6 cups a day of high cal kibble. 30% protein and 20% fat. Your dog may or may not need that, but my dog does. We are now moving toward RAW because it's just the same or cheaper for me because of the amount of kibble he goes through.
Good luck! Listen to your dogs body just like you would listen to your own!