Depends on what the question refers to:
The longest "running" event is the Marathon, but the longest event in terms of distance in the athletics is the Mens 50 kilometre walk (but you certainly aren't allowed to run in that).
Alternatively there are a whole series of events that have been included in every Olympics, therefore there is no single longest running event in terms of being at every Olympics.
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The longest running event in the Olympic Games in the marathon at a distance of 26 miles, 385 yards or 42.195 kilometers.