The words 'unwearied in that service' come from William Wordsworth's poem about Tintern Abbey. It is interesting that although the abbey is a beautiful natural and mysterious setting it also has has as its roots, a historical context as well. Obviously an abbey was a spectacular monument to spiritual ideas and was built with a religious intent - to the glory of God. We are used to William Wordsworth almost worshipping Nature as if it is a God, in all its magnificence and eternal beauty.
Now as the poet is growing in maturity and leaving his youth and childhood behind it only natural that he should contemplate his life and his service to humanity in holding aloft the magnificence of Nature for us all to admire as if a religion, yet he is also contemplating the idea of a further spiritual element to his life, and seems to be starting to acknowledging another Being. The lines refer to the fact that he sees praising Nature as a vocation to humanity and one he thinks he will never get tired of.