Have you done any running before? If you haven't, 5K is probably too long for a first time. The body takes a real pounding with running, especially the feet, and it takes getting used to. I started out with 10 minutes and built up gradually. The plan was to be up to a 10K by week 12. Although I didn't build up that fast, a lot of people can. You just have to listen to your own body.
Arch supports can be helpful but I think you'll have a lot better body feedback if you take your time and ease into the distance. The stretches you mentioned are a good idea, too. Don't forget to stretch your upper legs and hips as well, plus your back. There are some good references on Runner's World's website.
Good luck and enjoy! Running is a lot of fun, and it's very challenging.
Deb



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I find by taking 5 minutes to stretch all of the systems vs. just spot-targeting what irritates me, I care for the whole system and don't let a neglected part fail.