At least in NYC most of the 5Ks are really races. But you can certainly participate just to participate when you know you have no chance of winning.
The one on Castaway is just a fun run. Timing - and even completing the second loop - is on your honor. And unless you're doing it as part of the Challenge after a race weekend (that one has a fee and requires participating in the weekend at WDW) it's not even a true 3.1 miles. It's always been in the 2.8ish range when I've done it.