The first "miles to go before I sleep" stays within the boundary of liternalness set fourth by the rest of the poem.We may suspect as we have up to the point that the poet implies more than it says outright, but v cant insists on it the poem has gone by so fast and seems so straight forward .Then comes the second "miles to go before I sleep" like a penetrating gong; it can be neither ignored nor forgotten "miles to go before I sleep" THIS LINE SEEMS LIKE THE LAST "SLEEP" OF LIFE ie "DEATH"
I hope that I hav answered ur question ALL THE BEST!!!!!!!!!!!!
the first "miles to go before i sleep" means that he needs to get past the dark woods as he has made promise to someone and to fulfil his promise he has to travel quite far away. Another interpretation could be that his home is very far away and before he can sleep and relax, he has to cover a long distance.
the second "miles to go before i sleep". This talks about life being a JOURNEY and that sleep as DEATH. One may say that there is a long journey to go (i.e. lot many things to be done) before the final rest.
I hope i could help you!!! :) All the best!