I know of people that state length and times of walks etc and this is to do with the development of the bones, too much exercise at such a young age can be damaging to the delicate bones they have. I am sure somebody will state these times for you.
I think as long as Isla isn't doing strenuous, constant running on your walks then you should be fine, when we walk say 2 miles with a dog, they have probably done double that as there as here there and everywhere