Along

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Old English andlang, of West Germanic origin; related to long1.

wiktionary

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From Middle English, from Old English andlang, from prefix and- + lang(“long”). Doublet of endlong.

etymonline

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along (adv., prep.)

Old English andlang "entire, continuous; extended" (adj.); "alongside of" (prep.), from and- "opposite, against" (from Proto-Germanic *andi-, *anda-, from PIE *anti "against," locative singular of root *ant- "front, forehead") + lang "long" (see long (adj.)). Reinforced by Old Norse cognate endlang. Prepositional sense extended in Old English to "through the whole length of." Of position, "lengthwise," c. 1200; of movement, "onward," c. 1300. Meaning "in company, together" is from 1580s. All along "throughout" is from 1690s.