Hi, a good rule of thumb for large breed pups is to only walk them for 30 mins at 6 months old and keep them on the grass as much as possible to avoid damage to the joints. Keep to 30 minutes until they reach 12 months, as their growth will have slowed down by then. They can still run around in the garden and keep their mind occupied as mental exercise is just as good as physical
I have 2 large breed pups and this is what I have been advised by breeders and showers. Hope this helps.