Hey - congrats on the desire for the half marathon!! How exciting! I started running two years before I got pregnant. Haven't stuck with it since baby was born - life got a bit hectic. But need to get back into it now... I ran a half the year before I was preggo, and one when I was 7 weeks preggo. It is such an accomplishment... I also lost about 30 pounds around that time, so here are some suggestions:
Get a training schedule. A good one that I followed is Hal Higdon's - halhigdon.com. He provides some great schedules for any type of runner - from beginning to advanced, and for any type of race - from a 5K to an ultramarathon.
See if there is a running group for moms in your area - running partners are really, really motivating. They help you get out there, are good to talk to on a run, and will keep you motivated through your training.
If you are trying to lose weight, be really careful when you begin running lots of miles per week. You NEED to be sure you keep yourself hydrated, and fed. Don't let a strict diet or lack of calories get in the way of what your body needs to run. Also, if you are nursing, you obviously need to keep up those calories for milk supply. A book by Runners World - "Running during Pregnancy" has a last chapter about that.
Track your weight loss and running goals and accomplishments! It is so motivating to see what you have accomplished!
Good luck to you!!!