I think a 1/2 in 7 months is totally doable. A lot of 1/2 training plans are 12 weeks and you can find longer training plans that give you more of a build up in the 1st couple of weeks before longer runs. Some places to look for plans are on the rD site, they have Jeff Galloway plans(his plans are really long and start w/ really low mileage), Runner's World, Cool Running, I've seen plans in Shape and Women's Health magazines as well. My suggestion would be to look at as many different plans and choose the 1 that works best for you, b/c if you like it, you'll stick to it better. Another thing is if you see two plans you like or parts of 1 w/ parts of another, combine them and make a super plan.
Some random tips ~ don't increase your weekly mileage by more than 15% each week, b/c you'll be more susceptible to injuries. Your long run shouldn't be more than the total of the rest of your runs for that week for the same reasons. You don't need to do the full 13 miles before race day, most plans top out at 10/11 miles; the adrenaline race day will carry you through the last 2 miles. Don't sweat it if you miss a couple of runs, life happens and it's not going to set you back. Play around w/ fuel on your long runs so you go into race day knowing what works, race day isn't the day to try anything new. Race day, don't start out too fast where it comes back to bite you in the butt the last few miles; run your own race.