This is the 10186th most frequent Japanese word.
長き
"長き" means "length" or "long duration."
長きにわたる conveys 'extending over a long time', expressing the duration of peace in this context.
この地域では長きにわたる平和が続いている。
Peace has continued for a long time in this region.
長き年月 is used here as an adjective-noun phrase describing 'long years', indicating a lengthy passage of time.
彼は長き年月を超えても変わらない友情を持っていた。
He retained an unchanging friendship over the long years.
長き歴史 illustrates 'long history', showcasing continuity or longevity in historical context.