Harry: I certainly do, but they're supposed to be.
Niall: How often do you come to the gym for a workout ?
Harry: Three times a week at least if I can manage it.
Niall: No wonder you look so fit and trim.
Harry: Thanks. Stick at it and you can be the same.
If you stick at a task or activity, you continue doing it, even if it is difficult.
He became more and more irritated by her inability to stick at anything.