How long are these walks? Does she know the "come" command? You don't want to resort to picking her up too much, babying a mastiff can lead to bad behavior. Make sure she knows what come means, make a big deal out of it, lots of happy praise when she comes when called. She'll learn that when she comes it makes you insanely happy and that is the key, she will want to please you. Keep the walks short, lots and lots of praise when she walks with you. And lots and lots of praise when she pottys outside. She'll learn what makes you happy and what doesn't. All in good time