Shin Splints - Week before 5K! How to mitigate them?

So, I've been wogging (walking / jogging) for the past year and a half, ever since I started working on losing weight. In that time, I've run 3 5Ks, put in hundreds of miles of walking / training, and never had a problem. NEVER! Sure, I had shin splints back in high school (10 years ago), but nothing since I started running again last year.

Go figure, a few weeks before a 5K that I'm super excited about (racing my best friend, who is running his first 5K!), I start getting awful shin splints. I can barely do a mile, before they flare up and hurt like heck! In my case, the version I'm experiencing seems to be the over-stressing of the muscles in my lower leg. It feels like they cramp up so tight (almost like a charlie horse), that I can't even curl / extend my toes! It's painful, and it's killing my running.

I've tried running one day, taking two days off, and running the next day. But it happens again the next time. And the next time. And the next. I know that eventually it'll get better, as the muscles work themselves out, etc.

I've also got (fairly) new shoes. The same shoes I bought brand new last year, that I've only run in, have served me well. But I'm getting shin splints, even when wearing them. So I had purchased another pair of shoes fairly recently, and tried running in them. Same thing. So, I'm not sure what happened in my life to make the shin splints start to suddenly become a problem.

So, the long and short of it, I guess, is trying to figure out what I need to do in order to be able to do the 5K tomorrow, and not feel like my legs want to fall off when I'm done. It's frustrating, because I've finally started to get over the breathing hurdle (I sounded and felt like a 2-pack-a-day chain smoker for years), but now my legs are restricting me! Frustrating!

Anything I can do tonight (stretches, diet, etc) to help curb it for tomorrow? We're going to go walk a few miles tonight, just to stay limber for tomorrow. How about potassium? If my diet was short on that, could that possibly be a problem? Should I scarf down a banana or few between tonight and tomorrow? Anything else?

Any ideas would be appreciated. Thanks!