Santiago is, as the title suggests, an old man. He's been a fisherman for his entire life, and he has learned to overcome any obstacles which face him. In The Old Man and the Sea, Santiago's greatest obstacle is not his solitude, though he does have to deal with it to a greater extent than usual because he's gone for so long.
While he's at sea, Santiago needs to keep himself strong in every way. Physically, he eats the tuna he catches, tries to rest when he can, and does his best to protect his hands and back from the strains of the line. Mentally, he reminds himself of the great Joe DiMaggio and his bone spur. Emotionally, he tries to stay focused on the present, despite his reveries and daydreams. This is where he fights his solitude, and he does so by talking to the living creatures around him (the birds, the fish, the sharks) as well as bolstering his energy by talking to himself (his hands, his back, his mind). This is his only defense against the solitude of being adrift in the ocean an at the whim of a giant fish. The greatest danger of his solitude, of course, is bringing in the fish by himself--which he does.