Odds of Hanging On
The first test is the hardest: can you last a day on the ship without hurling? Really, that's the biggest issue to overcome. Sure, you'll have to put away your ego, deal with ranked officers barking out orders, and learn the hard way why it's called a "poop deck," but you won't get very far at all if your sea legs are a little too wobbly.
After you've passed that first test, you just have to apply yourself and work harder than you thought possible. If you can push yourself, deal with the exhaustion (plus the smells), and work well with others, you'll be able to hang on to this career for as long as it takes to reach the horizon (and that's forever, by the way).