On all the best dog food for Mastiffs, you’ll see that little disclaimer “with adequate exercise...”. What an adequate amount of exercise means will vary depending on your puppy’s life stage.
Puppies will generally get enough exercise on their own, provided you give them enough space to do so. This means that if you have a large yard and it is possible to do so, let your Mastiff puppy run around at his leisure. You may want to consider a wireless dog fence, but it’s never a good idea to leave your Mastiff tethered if you can avoid it.
If you haven't got a yard, it might help to take your puppy to the park for 30-40 minutes each day, twice daily. This way, when he's tired, he can stop playing by himself and rest. You should try to avoid overly vigorous exercise for these soon-to-be giants because their joints and bones are not yet fully developed.