I had the pleasure of viewing this conversation on Facebook. Two people I went to high school with recently got into running, and have been running all of the local 5k's or similar back in then tristate area.
This was the conversation that happened under a picture that they posted.
Runner #1: I ran the race in xx:xx and placed 13th out of 75 in my age group!!! I am starting to get the hang of this marathon thing :)
Person #1: You flew by me at mile 1 and I never saw you again
Person #2: I love your matching shoes!
Runner #1: You should join us for one. We are doing the race in (city) next weekend.
Person #2: I can't because i have to work.
Runner #1: At the end of the month we are doing the Color Run. We are also running two more marathons in July in the area.
It amazes me how Runner #1 interchanged the words 5k and marathon, acting like they were the same thing. Personally, I have always known the difference because I grew up around running. Marathons were a great distance, 5ks a short run.
Having trained for both 5ks and marathons before, I can, as well as anyone else, say that there is a gigantic difference. It takes a few weeks to train for a 5k, but months and hours upon hours to train for a full marathon. It would be like saying that you got a home run in baseball when really you were walked. You still got on base, but it was only to first. You did well, but could have gone farther.
This isn't to say that running 5ks is easy. I think they are incredibly hard if you run them all out. However, they are nothing like a marathon. 5ks are done in 30 or less minutes, while marathons take a few more hours to finish.
Are there other things that you often hear people confuse? I have heard 5k and marathon before, but also biathlon and duathlon often get confused.