The biggest challenge that Santiago has to face in his fight with the marlin is that he's not getting any younger. Santiago's too old to be engaging in such a long, grueling battle with one of nature's aristocrats. But he rises to the challenge, seeing the marlin as a formidable opponent, well worth all the time and effort he puts into catching it. Strong, relentless, and full of tricks that can throw even the most experienced fisherman off the scent, this is a worthy adversary indeed, and Santiago's ego cannot resist the challenge.
Although the old man eventually prevails in this epic battle of wills, it takes a lot out of him, both physically and mentally. By rights, this really ought to be Santiago's last fishing expedition, but as this is a man with such an active personality, we can't really imagine his getting out the pipe and slippers just yet.
Once he's caught the big fish, Santiago then has to deal with the additional challenge of getting his catch home in one piece. But that's easier said than done with a large school of sharks circling round his boat. Santiago somehow needs to fend off these fearsome creatures if he's to bring his marlin safely home. Unfortunately, he's unable to do so, and his prize catch is reduced to a bag of bones by the greedy sharks.
Maybe when he was a lot younger Santiago would've been able to overcome this particular obstacle. But now that he's rapidly heading towards his twilight years, this is just one challenge too many for the aging fisherman.