Runners: What do I do?

I did the 5k and then moved from there to the 10k. I was running so much more doing the 10k intervals that I could still pretty easily run a 5k. I think you should be fine, but if you are worried about it add a 30 minute run here or there.

So, the C210K program has you start back from the beginning (i.e. the "couch" part), I'm assuming? Why not start a 5k to 10k program? Since you're already running 5k distances, it doesn't seem to make much sense to start downgrading back to beginner levels (pardon me if that's incorrect, I haven't read the C210K program). I am currently training for a 10K as well, and am using this program, where I am running intervals on the shorter days (i.e. 2 minutes normal pace, 1 minute sprints), and regular pace on the long runs: http://www.halhigdon.com/training/51122/10K-Novice-Training-Program. There is also this program which is similar, but it involves running a tad more often than every other day: http://www.coolrunning.com/engine/2/2_4/138.shtml.

I say start training now from your 5k point, not "couch" level, and your 5K is going to seem so much easier in comparison once you've got a few longer runs and some interval training under your belt! Good luck with the race, the first one is so exciting!!