Good for you for taking on the 13.1 mile challenge. Just a word of caution here is throughout your training, really listen to your body. It takes the body a good while to adjust to the rigors of running. It's not just your cardio and muscles, but your joint, ligaments and tendons, too. Most people can get the cardio rather quickly, but the rest of it takes time and smart training. Push the mileage too soon or try to take on too much and you will get injured. The fact that you still have 80 lbs to lose means that you really need to slowly ramp up your miles in the beginning.
I'm not trying to pooh-pooh your goal, I'm just saying to do it smart. Most people don't even start training for a HM until they have been running consistently (at least 3x a week) for a year. You might want to look into the Galloway running program. It's a run/walk that makes it easier to do the distance.
Best of luck on your goal. I know you can do it!
"God gave you your body as a gift, so you should take care of it." - My Mom