Stick at it

Niall: Don't you find these rowing machines hard work ?

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.

Explanation :

If you stick at a task or activity, you continue doing it, even if it is difficult.

Example :
He became more and more irritated by her inability to stick at anything.