Spending your whole life searching? Continuing to search even when your strength is gone and you're about to die? That's perseverance if we ever saw it! The knight's determination in "Eldorado," however, isn't all fine and dandy. In fact, it more or less kills him. And he still never finds Eldorado. Throw that in with the fact that he's all alone, with only a "pilgrim shadow" for a friend, and you've got a pretty unhappy picture of perseverance. A for effort, though!
There's a fine line between perseverance and obsession, people. If one pursues something too aggressively one risks ending up like the knight and missing out on what life has to offer.
Eldorado, shmeldorado. It doesn't matter whether or not the knight actual finds the lost city of gold. It's important that he perseveres to the end.