Exercise and plenty of it will build up stamina. Don't worry about going really fast, its more about the distance, just so long as the dog isn't just walking but she doesn't need to be running full pelt to build the stamina up, a steady jog will be OK as long as you arent stopping every 5 minutes for a breather.
Think about those that run marathons, they've got incredible stamina but don't run the marathon like a sprinter would run the 100 meters.
How many miles are you taking her each day?