A lot of things could be factored in. How much exercise is she used to getting? Is it unseasonably warm where you are? Is she overweight? Has she been checked by a vet to make sure she doesn't have any health problems? Is she otherwise active and running around?
My 6 year old pit bull was panting heavily after about ten minutes on our walk today. We haven't walked hardly at all since the spring, and it got a lot warmer than I was expecting it to. By the end of our walk, which I cut short at just over half a mile in twenty minutes, I thought I was going to have to carry her the last hundred yards or so back to the car.