It's possible she lost her endurance, but maybe she's in pain. After all, she did just have surgery on her foot. Even if she is all healed up, she may just not be the same. Maybe she has scar tissue that causes her pain. I don't run as fast when the pain from the neuromas start to show up. I also keep thinking about what I learned from reading BTR about animals only being able to run for shorter distances at a time than us. Do you feel maybe a vet visit is in order? Whether the dog is ill or not, they can always answer any questions you might have.
