Your pup is too young to be forced to run for such a long time. They should only run around when they want to, and be able to choose to stop when they get tired. Forced runs at a young age can cause damage to the growth plates in their joints. Wait until the growth plates close (usually 18-24 months old for a large dog) before running with them, or running them on a treadmill. You could have the dog walk on the treadmill instead, or go for longer walks.
I HATE retractable leashes! I've seen a puppy wind itself around its owner while on one, then take off, yanking the person off her feet. Then the puppy was loose and running down the street and into traffic - scary! I also don't like approaching a dog who's on a retractable leash. The owner doesn't have control of such a dog b/c they're too far away from them. I always cross the street to the other side when I see a dog approaching who's on a retractable leash. I also think they train the dog to pull, since they have to pull more of the leash out of the holder.
While on a standard leash, try stopping and refusing to move everytime your dog pulls. Yes, it might take FOREVER to get anywhere at first - practice in your own yard for the first few times. You have to be consistent with this. Once you stop moving, don't have any reaction while your dog pulls; no yanking, no yelling. Eventually, the dog will get curious and turn around to look at you (thus loosening the leash), wondering why in the world you're not moving. At that time, give him a high value treat (TINY pieces of hot dog or boiled chicken work well for many dogs). Take another few steps (the dog will probably start pulling again). Stop again and wait. Treat when the dog eventually turns around to look at you, loosening the leash. Consistency is the key. This may take DAYS before you start seeing results - be patient. The dog has to realize that he only gets to walk when he's not pulling. Making the dog stop every so often and sit/stay is good training, too. Again, treat when you get a positive result.
You can also change direction often, sometimes after only a few steps. Dogs tend to start pulling more when they're going in the same direction for awhile. Again, you may want to start practicing in your own yard. Let the dog know you're going to change direction before you actually do it, so you don't yank his neck. You can say something like, "let's go this way", or something like that.
I use a Gentle Leader on my fiesty westie, because she's quite a puller, even with a choke chain or martingale collar. She didn't like it at first, but she's learned that it means a walk, so she's accepting it now. You can adjust it so that the dog can still breathe and pant just fine. You want to adjust it tight enough to stay on, but still have the dog be able to open their mouths to pant. Be prepared for your pup to pitch a fit when it's first put on (some dogs will even do fish rolls trying to get it off at first - lol!). Just keep walking. Don't yank on the leash when using the Gentle Leader. This collar puts pressure on the dog's sensitive nose, so you want to be careful.