Pip ascertained that his "job" while with Miss Havisham was to do whatever she said to on the days that he was there. On most days, she wanted to watch him play cards with Estella, and be available for Estella to make fun of him and practice hating men.
On this her birthday, his role had changed. She asked him to "walk" her. As she didn't get up out of her chair very much, he understood his role was to be her escort helping her walk around the bride-table she had set up in the room. She leaned on him and they went around the table many times at a very slow pace. For a boy his age, this certainly would have been work.