As with other epic modern-day adventure race series, the fabled origin of Seward, Alaska’s annual Mount Marathon — held every July 4 — began with beer and a bet: who among us can run from the streets of downtown to the top of the 3,022-foot mountain crowding Seward’s skyline, then back to the bottom the fastest? What about within the hour? While the footrace race has evolved massively since the port town’s pioneer miners took...