"that extends considerably from end to end," Old English lang "long," from Proto-Germanic *langgaz (cf. Old Frisian and Old Saxon lang, Old High German and German lang, Old Norse langr, Middle Dutch lanc, Dutch lang, Gothic laggs "long").
The Germanic words are perhaps from PIE *dlonghos- (cf. Latin longus, Old Persian darga-, Persian dirang, Sanskrit dirghah, Greek dolikhos "long," Greek endelekhes "perpetual," Latin indulgere "to indulge"), from root *del- "long."
The adverb is from Old English lange, longe, from the adjective. No longer "not as formerly" is from c.1300; to be not long for this world "soon to die" is from 1714.
The word illustrates the Old English tendency for short "a" to become short "o" before -n- (also retained in bond/band and West Midlands dialectal lond from land and hond from hand).
Long vowels (c.1000) originally were pronounced for an extended time. Sporting long ball is from 1744, originally in cricket. Long jump as a sporting event is attested from 1864. A ship's long-boat so called from 1510s. Long knives, name Native Americans gave to white settlers (originally in Virginia/Kentucky) is from 1774. Long in the tooth (1841 of persons) is from horses showing age by recession of gums. Long time no see, imitative of American Indian speech, is first recorded 1900. To be long on something, "have a lot" of it, is from 1900, American English slang.