I've seen several posted on Facebook. I did mine this morning and "signed up" with RunJunkEees on Facebook. I walked/ran 5K for Boston this morning, and am doing it again with a friend Thursday evening!
The local running shop in my town is hosting one next week. We're getting together to do it, but you just bring a donation and meet up and run-no formal registration or anything. Love that people are raising money for this through running!!
I posted a link on my blog but there's a website where you punch in the miles you ran today and where you ran them and it creates them all in a google doc. So far they're at over 2200 miles. It's so cool seeing everyone doing it. I love seeing all these runners come together in unity.