Golden with stamina issue?

Species: dog

Age: 10.5 months

Sex/Neuter status: female, not yet spayed

Breed: golden retriever

Body weight: 65+ lbs

Hello, I'm wondering if I should be concerned with my puppy's stamina. She is fairly active; I take her on one 30 minute walk plus a few 15 minute walks daily during the week, and hours long outings on the weekends. She is good on her walks and can go for miles/hours with no problem (though she will lay down and sleep/rest after).

The problem I notice is that she gets tired fairly quickly 10-15 minutes after playing/running hard with other dogs (playgroup, dog park). She will go hard for short spurts, but then will come and sit down by me panting quite hard. She also gets very thirsty and foamy at the mouth. She is a field golden so I would expect her to have a bit more running stamina, but maybe I am wrong? I'm not sure if it is that she hasn't built up enough running endurance (we do mostly leisurely hikes/walks), and she is slightly out of shape, or if there is a underlying health issue I should be concerned about?

She does not cough, or exhibit any other worrying symptoms besides the panting and fatigue, and her heart hasn't exhibited any murmurs under normal vet visits. Within 15-20 minutes (sometimes sooner) she is usually back to normal breathing, no panting.

She is not yet spayed and just completed her first heat mid March.

I would greatly appreciate anyone's opinions on whether this is still in the realm of normal puppy behavior or if I should be on the lookout for additional health concerns?

Thanks!