Push hard enough to surprise the version of you expecting surrender.

There is a version of you with a fixed idea of when you will stop, and it usually announces the stopping point well before your actual limit. Going past that announcement once, deliberately, changes what you believe about the limit, and the belief is doing more of the work than the body or the mind. The surprise is the point.

Pick something where you know roughly where you give up, and go clearly past it a single time. Not to exhaustion and not as a habit. Once is usually enough to move the internal estimate, and the new estimate is what you will be working from afterwards.