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.
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.
For men it is the 50 kilometer walk and for women it is the 20 kilometer walk.
Qieyang Shenjie China (CHN)
Olga Kaniskina Russia (RUS)
Wang Zhen China (CHN)
Chen Ding China (CHN)
Jared Tallent Australia (AUS)
Si Tianfeng China (CHN)
Erick Barrondo Guatemala (GUA)
A 5 km walk event has never been a part of the Summer Olympics. There was a men's 3 km walk at the 1920 Games in Antwerp and a men's 3.5 km walk at the 1908 Games in London.
The bronze medalist in the Men's 50 Kilometer Race Walk event at the Rio 2016 Olympics was Evan Dunfee.