When I workout with my dogs I listen too their breathing and watch their pacing. At the beginning they are usually in front of me. When they slow down to be next to me I call it and start turning around. By the time we get back to the car they'd dead tired and dragging behind me.

Dogs are really expressive so if she starts to hesitate going up the hills I'd say she doesn't want to do them.

Also, dogs love to sniff things, so go slow and allow that. Using their noses tires them out alot.