O Captain! my Captain! our fearful trip is done; (1)
The trip is described as “fearful,” which implies that the suffering has been more mental than physical.
The ship has weather’d every rack, the prize we sought is won; (2)
Before the speaker knows that the captain is dead, he can accept that the suffering that was endured in the voyage was totally worth it because the mission was ultimately a success. In other words, the juice was worth the squeeze.
While follow eyes the steady keel, the vessel grim and daring: (4)
Look at the portrait of any president before and after his term in office and you can see how much the suffering that comes with great responsibility can have a physical effect on a person. White hair and wrinkles, much?