a bad day for running

I was excited to go running today after getting a new pair of Mizuno Mercury's. Back in early November, my beloved pair of Mizuno Mercury's was getting worn out, and the local running store convinced me to buy a pair of Mizuno Alchemy, which have been nothing but trouble since I first wore them for runs longer than 5 miles. Blisters, foot pain, you name it. So despite the expense, I broke down and got the new model of the old Mercury's.

I was trying to cross-train and rest after Saturday's 12-miler, thinking that two days without running should have been enough of a break. And still the same problem-- despite walking beforehand, stretching carefully, and starting off with a slow jog, I could feel what I think is a shin splint on the muscle slightly to the outside of my shin-- that "heavy legs" feeling again. I'm so upset about this-- no Achilles tendon problems now, but shin splints? With the marathon less than a month away, and our last 20 miler this weekend... Someone told me about an exercise that involves leaning your back against a wall and lifting your toes up toward your knees, but aside from that, I don't know what to do. Does RICE really work?

I have been in a funk about this all day.