A Mastiff due to its size will cost you more money at the Vet's office also. Heartworm medicine costs more, shots can sometimes be more costly, etc. A Mastiff needs obedience training. It is imperative that obedience training be done. The obedience training must be the positive reinforcement type.
Mastiffs respond well to love, praise, and especially treats.
Exercising a Mastiff is not as difficult as exercising one of the various sporting breeds who seem to have endless energy. A Mastiff is happy to go on 2 walks a day of about 20-30 minutes. They will not jog and should not be asked to as it can be very difficult on their joints. About a mile or so walk twice a day is enough unless it's an older Mastiff, then play it by ear. Their exercise can be walks with you around the neighborhood, hikes in the forest, swims in the local lake, or chasing a soccerball.