Santiago often strategizes, reflects, and prays while struggling with the fish. After a time, Santiago grows fatigued. Though he will return to strategizing, reflecting and praying, he tells himself "only to endure" as a way conserve energy and as a way to simply cope with the situation.
We can say then that Santiago is telling himself to hang on, to keep calm, and to save his energy. Santiago knows that the fish is stronger than he is.
He is a great fish and I must convince him, he thought. I must never let him learn his strength nor what he could do if he made his run.
Discipline and intelligence are Santiago's advantages. One aspect of his discipline is seen in his advice to himself to endure.