Stop the walk. A dog can keep moving even when heat is becoming dangerous. Get them into shade or a cool space, offer water and call a vet urgently if panting is excessive, they seem weak, unsteady, confused, drool heavily, vomit or collapse. Heatstroke is an emergency. Begin cooling with cool water and airflow while arranging immediate veterinary care. Do not use ice or cover the dog with wet towels, and follow veterinary advice. Do not wait to see whether a walk home helps.
Do not judge safety by a fixed distance, the dog's enthusiasm or the air temperature alone. Humidity, sun, effort, breed and health all matter. Dogs Trust warns that heat illness can happen on ordinary walks. The hot-day routine article offers a way to meet your dog's needs without the usual route when weather makes it unsafe.
Once the dog is safe, rethink the next outing. Choose a cooler time, shade and short toilet access. Carry water and plan an easy return. If this panting was new or out of proportion to conditions, ask your vet whether a health problem also contributed. Do not use a longer walk to find their supposed limit.
Today, end the hot outing and assess the dog now. A dog asking to continue is not a medical clearance.
Sources: Dogs Trust on heat risk during ordinary walks, VCA on heatstroke as a veterinary emergency.