I think this is rather easy to dig into...but I do have a degree in lit and sitting around talking about poems was much of what we did.
I'm not expert on Longfellow, however...
He was a direct descendant of people who came from England. These puritans were an odd bunch if you ask me - they claimed all kinds of torture in the old land when it seems they were really interested in becoming slave holders and capitalists.
Longfellow, it is claimed, wanted to create an 'American' mythology. If you've ever read something like 'The Odyssey' or 'The Illiad' you would pick up on some things here. Both of those, ummm works (all billions pages of them), make you feel extremely rooted to humanity and at the same time it's quite uplifting to know that you are connected to this greater thing. I think that's what he's doing here.
The Puritans allegedly believed in hard work ("it's hard work" says George Bush). A faith-based work initiative if you will (oh god, I'm starting to annoy myself). Anyways, this is certainly a faith-based poem initiative.
Perhaps he was telling his students (I think he taught at Harvard) to believe the beliefs, work hard, be humble and pious and holy and such "with a heart for any fate." Oh, what's his name? Ah, Ezekiel - your resident crazy bible man who was a leader and a 'hero'.
The second to the last line get's me, is this for real?
Still achieving, still pursuing,
He should have been writing copy for MS! For an even more obtuse angle on that read the lyrics of "Empire" by David Byrne (yes, this a cynical, cynical song).
http://www.talking-heads.net/lyrics_backwards.html#4
Don't know if that helps. It was kind of fun doing that, even if it sucked. I haven't done that in many years. Perhaps I'll so some more now.