Well, where did you get the notion to keep Missy in a muzzle?
The video above, i myself don't see anything wrong, except that you are allowing Missy to pull so hard. I can almost hear Missy rasping once or twice, as the tension on her collar gets real hard pressure. Missy doesn't look aggressive to me at all, in the video above, but just excited, and mayyybe pulling to get away from the other dog barking away there. I would have asked dog to sit, look at me, slow blink and yawn to him, also reward that "look at me" and see if that helps dog calm down a bit. (works for my truly aggressive dog, anyway, most of the time)
It's an interesting case, you don't often hear about a dog who has never ever ever met and played with another dog. Amazing, of all your friends, all your neighbors, all your relatives, no one owns a dog. wow. Because your dog has never ever played with another dog in his life (?) with the exception of your friends new puppy, it is hard to tell if your dog is or is not dog-aggressive.
It is going okay with your friends' puppy, or no? Has Missy played with the puppy ? Puppies is hard thing to tell by, cuz even many 'normal' dogs scold out hyper puppies now and then, some adult dogs dislike puppies, but are otherwise 'normal' dogs.
Sometimes the dog complaining IS right!
AT the park, the ears fwd stare doesn't
sound friendly, but the lack of stiff tail/hackles up, it's hard for me to sort out.
but i still don't think Petsmart is the best place to try to find out. I really don't think Petsmart is the place to take Missy next, but, i think trying to locate owners of
friendly dogs, and asking them if your dogs can play for a lil while, might be good idea. Of course, you are young, not a good idea for you to be alone around strangers, but, if your mom came with you, etc, might be good way to check out if Missy is or is not okay around dogs. Maybe your mom has coworker with a dog, someone with a dog to find out how MIssy is around dogs.
Or check out dog training classes, and explain to teacher, your dog has never in his life interacted with other dogs, and you want to learn how your dog gets on with other dogs.<---really, this might be stellar idea, so in case your dog DOES have some issue, there is a pro there to help approach this in safe way. Maybe have teacher set up an hour of dog play for your dog and some dogs she KNOWS are well behaved, friendly dogs.
Even though there is no proof your dog is aggressive dog, (so far as i know) i'd still look for a trainer who has experience with behavior issues, not just tricks-training, and one who is POSITIVE ONLY, last thing you want is some trainer yanking MIssy around by her neck.
unless there is something i'm missing, because i don't yet understand why you keep Missy in a muzzle.
Oh, also, is Missy friendly towards unknown humans?
Also, so great you are getting Missy in shape for the 5K, too. Still, unless you are going to run the 5K in 15 second increments, you'll have to keep working towards longer amts of time running.
What a clever way to do it, though, Southerngirl, jog-walk-jog-walk! I call that "wogging" hee hee! Great plan, great way to build up your dog!!! Kudos!