My Mastiff loves to rough house and will oft times completely ignore my wifes commands and bark at her as if he is telling her "Please?! I got some gonads you dig? I'm male Mastiff, back off!" He gets really rambunctious but stops short of actually challenging me, as we have had at least one "biblical" confrontation. What I try and do to alleviate this is take the dog on runs and play tug. A tired dog is a good dog. Yes they will get more fit and you will have to get more creative or work the dog harder. However, don't forget it is a Mastiff and not a pitbull you are dealing with here. Mastiff's will only run so much :) They are inherently lazy creatures. So it's not impossible to tire them out. If you are a former boxer you may probably still do some street work, maker her jog with you at a leisurely pace for a good 3 to 5K. I normally drag weight my dogs for fitness, but unfortunately this is the biggest dog I have owned and he is too big for my harness. So I have to order a new one when he stops growing in order to use this method. Don't listen to Platz, I once paid him to come and give me some dog training and he beat me in front of my dog instead of correcting the dog. Platz is a mean old violent drunken Rottweiler breeder, you have been warned!