Correct my answer - which version of the sentence uses correct parallel structure?

saving is the key to a comfortable old age, and not saving is to be the key to an anxious one.

saving is the key to a comfortable old age, and not saving is the key to an anxious one. ^^^

saving is the key to a comfortable old age, and in not saving is the key to an anxious one.

saving is the key to a comfortable old age, and not to save is the key to an anxious one.