Dave Murray was playing with Adrian Smith long before Murray started playing with Harris.
Even beyond knowing each other since their youth, Murray and Smith played together in Evil Ways from 1972 to 1974, a band which later mutated into Urchin. Murray did not join Iron Maiden until after his time in Evil Ways.
Finally, the question of who has "stuck with Steve" the longest is IRRELEVANT. Steve Harris may be good, but he's no Adrian Smith. Only playing with Adrian Smith matters.
You poor, deluded infidels who deny the absolute lordship of Adrian Smith are only hurting yourselves. If you persist in your heresy, you will suffer in the afterlife. You will be denied the privilege of being one of Adrian Smith's guitar techs in Heaven. Instead, you'll wind up in some forsaken purgatory, changing guitar strings for Linkin Park and Slipknot.