Longevity, longitude, and other words with the root long- (or altered spellings of the root) are derived not from long, which stems from the Old English adjective lang, but from the Latin equivalent longus, which shares its Proto-Indo-European ancestry with the Germanic cognate. This post lists and defines words that stem from the Latin term.
Longevity, from the Latin adjective longaevus, means “long life.” Longitude, meanwhile, in general means “length” or “height,” but it usually pertains to horizontal distance on the surface of Earth or any sphere (and to a line marking such a distance). The adjectival form is used in the measurement sense but also pertains to long-term research studies and to a vehicle engine that runs a lengthwise rather than crosswise crankshaft.
Elongation and prolongation both refer to extension, but the former applies in a physical sense, while the latter sense is chronological. The verb forms differ, too: They are, respectively, elongate and prolong. Oblong, meanwhile, describes something that is longer than it is wide, though it can be used as a noun as well as an adjective.
English borrowed longeurs, a word describing a tedious passage in a book or a play, from French.
Two obscure words derived from longus are longanimity and longinquity. The former word means “forbearance” or “patience”; the element animity is from the Latin noun animus, meaning “mind” or “soul,” which is the source of animal. The latter is an archaic synonym for remoteness.
Several words with disguised kinship are lounge, from the French verb s’allonger, meaning “lie at full length”; lunge, originally a fencing term meaning “sword thrust” (from the French noun allonge) with the extended meaning “sudden reach or rush”; and purloin, meaning “steal” (from the Anglo-French verb purloigner, meaning “remove”; the connection to longus is the idea of delaying something or moving it far away).
Along, belong, and length are, like long, of Germanic origin, as are compound words such as longbow, longhaired, and longtime, as well as headlong, lifelong, and so on.