I ran my first rD 5K in 2013..Food and Wine, I was in my early 60's. I started training but summer came and it was just too darn hot! So, I basically did it with no training. I'm in decent shape though. Finished with about a 14 min mile. I was really worried about being 'swept' from the course, but when I was about a mile from the end, I was able to look way behind me....there had to be a thousand people behind me!!! No longer concerned with being swept!!! Corrals are really assigned according to when you register. Those that are true runners, with proof of times, tend to get put in the front, corral A. I know that the first time I ran, the guy that finished first in the 5k went on to finish first in the half marathon. We ran in AK...as I was just running past the Tree of Life, the more elite runners were running past on the other side...about 15 mins ahead of us!!! Just a wee bit intimidating!! The second time I ran, I ran a bit faster. Then, last year I ran at the age of 68, with my 24 y/o dd. She doesn't run anywhere...well, maybe to a bottle of pinot grigio!! So, there was a lot more walking than I usually did..used to do a run for a few minutes, walk for a minute or two. But, we still finished in under a 14 min mile.
So, go for it. Just try to start a lot of walking beforehand. If you intend to really run it, then you need to train. But, it is entirely possible to walk/run it, and still finish in the middle.