i have two ideas about this line's meaning:
one is he thinks he still have a lot of things to do before he sleeps or dead, because of the line "but I have promises to keep" (14)
the other one is he thinks he still need to walk miles away for death, which is because even his horse think it "queer" (5), so the reason that he goes into the wood is because he wants to die in there.
Anyway, the repetition may be just give people more space to think about it or just emphasizing it.