Although Mr. Longfellow lived in the 1800s, I think his words about achieving apply today as well. 
The doing can be anything that your heart desires as long as you are achieving, pursuing, laboring, and waiting. I think he means let fate be what it might, whether you face failure, obscurity, or fame and success. It’s the achieving and pursuing that is worth doing.
What do you think Mr. Longfellow means?