maybe your sessions aren't long enough? Do they seem to still have energy? Do the same routine. When you come in teach your dog a place you want them to go, and once they stop panting and get back to regular breathing release them from their place. They will eventually learn that's what you want from them when play stops. you could also have a treat or dip and antler in peanut butter and when you come in give them that as the signal play is over.
Also, welcome to GP! Take a look around and let us know if you have any questions. post up a picture of your dog. And a vet isn't going to know what breed a dog is just by looking at it. They can make and educated guess but unless you know how the dog was bred, you will never know what breed they are.
"There are two temptations to which people fall prey with dogs. The first is to think that biology counts for everything. The second is to think it counts for nothing"
- Tom Junod
“When you KNOW better you DO better.”
― Maya Angelou