I love this idea! Especially as am addicted to Twitter! In my first half marathon I set the message tone to my daughter saying "come on, Mummy!" so I could hear it if I had my headphones in and sent out a message to friends asking for messages around the 6 - 10 mile mark!! Is there a version for iPhone?
I wore knee length capris and wish I'd worn full length tights as I've still got the scars on my legs now so cover up your skin. But don't wear loose clothes as there are a lot of water and mud obstacles and you don't want to get weighed down too much. I wouldn't bother with gloves - so long as you can run 12 miles you'll be fine! Enjoy!