“The life of man is a long march through the night, surrounded by invisible foes, tortured by weariness and pain, towards a goal that few can hope to reach, and where none may tarry long.”

1963, UN speech
Context: The task of building the peace lies with the leaders of every nation, large and small. For the great powers have no monopoly on conflict or ambition. The cold war is not the only expression of tension in this world — and the nuclear race is not the only arms race. Even little wars are dangerous in a nuclear world. The long labor of peace is an undertaking for every nation — and in this effort none of us can remain unaligned. To this goal none can be uncommitted.

Source: Introduction à l'Étude de la Médecine Expérimentale (1865)

Due to German student movement leader Rudi Dutschke, who coined it in 1967 as „Der lange Marsch durch die Institutionen“.
See Strategy, Hegemony & ‘The Long March’: Gramsci’s Lessons for the Antiwar Movement http://carldavidson.blogspot.com/2006/04/strategy-hegemony-long-march.html, by Carl Davidson, April 06, 2006.
It was popularized in the protests of 1968, and Dutschke’s posthumous 1980 work is titled Mein langer Marsch (My long March).
See Marsch durch die Institutionen at German Wikipedia for extensive discussion.
A reference to the Long March of the Chinese Communist Red Army in 1934 & 1935; note that Gramsci died in 1937.
Various corruptions include “through the culture” or “slow march”.
Widely attributed to Gramsci, Joseph A. Buttigieg http://english.nd.edu/faculty/profiles/joseph-a-buttigieg/, the editor of the English critical edition of Gramsci’s Prison Notebooks asserts that the phrase does not originate with Gramsci.
Footnote 21, page 50, reads: [“long march through the institutions”²¹] “This phrase is not Gramsci’s, even though it is ubiquitously attributed to him.”
[10.1215/01903659-32-1-33, 0190-3659, 32, 1, 33-52, Buttigieg, Joseph A., The Contemporary Discourse on Civil Society: A Gramscian Critique, boundary 2, 2010-06-30, 2005, http://boundary2.dukejournals.org/cgi/pdf_extract/32/1/33]
The idea is connected with Gramsci's theory of cultural hegemony, but does not originate with him – he called the concept a “war of position”.
Misattributed

Main Street and Other Poems (1917), Apology

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 49.

Vittoria Colonna.
Bartlett's Familiar Quotations, 10th ed. (1919)

Source: Empire novels (1950–1952), Pebble in the Sky (1950), Chapter 5 "The Involuntary Volunteer" (p. 57)

No. 8.
Maxims and Moral Sentences

Epistulae ad Atticum (Letters to Atticus) Book IX, Letter X, section 3
Often paraphrased as: Dum anima est, spes est ("While there is life there is hope")
Compare: "While there's life there’s hope, and only the dead have none." Theocritus, Idyll 4, line 42; as translated A. S. F. Gow