We can infer that Roger, while executing his plan, expected a large woman like "Mrs. Luella Bates Washington Jones" to be no great sprinter, therefore unable to run after him once he had her purse in hand. We can infer that Roger did not expect to be kicked "right square in his blue-jeaned sitter" nor did he expect to be hauled to his feet by "his shirt front" and shaken "until his teeth rattled" (nor did he expect to land flat "on his back on the sidewalk"). We can infer that Roger expected a helpless woman, an easy-to-snatch long-strapped purse of normal weight (which hers was not as it had "everything in it but hammer and nails") and a "full blast" escape.
We can only infer what Roger expected about Mrs. Jones's toughness because the story begins with action in medias res, or in the middle of events that began while we were out of earshot, so to speak. Before the story even begins, Roger has thought over what resistance he expects, what toughness he expects, what he is going to do, whom he is going to target and how he is going to make his move, all this before he ever grabs the strap of Mrs. Jones's too-heavy shoulder-strap purse (if Roger acted impulsively on the spur of the moment, he would have no or many fewer expectations).
In the story's time-frame, Mrs. Jones turns out to be tougher than Roger expects by courageously turning the assault against Roger, by hanging relentlessly on to his shirt front and by forcing him to behave like a well-raised boy while he is in her company.
“If I turn you loose, will you run?” asked the woman.
“Yes’m,” said the boy.
“Then I won’t turn you loose,” said the woman. She did not release him.
Mrs. Jones also turns out to be tougher than Roger expects by making him wash his face and comb his hair, by making him eat some supper with her and by teaching him that even though someone does wrong, they have a chance in life to learn to do right. Being tougher than Roger could have earlier expected Mrs. Jones to be, she teaches him this by the anecdote she tells of her own life and by freely handing him the money for the shoes.
The woman was sitting on the day-bed. After a while she said, “I were young once and I wanted things I could not get. ... I have done things, too, which I would not tell you, son—neither tell God, if he didn’t already know.” ... When they were finished eating she got up and said, “Now, here, take this ten dollars and buy yourself some blue suede shoes. And next time, do not make the mistake of latching onto my pocketbook nor nobody else’s .... But I wish you would behave yourself, son, from here on in.”