You don't say how long you have been running, but for the most part, you should be running about 3x a week consistently for a year before even starting half marathon training. However, if you had been running for a year or more before your 4 month hiatus, then you probably can start training later this year, as long as you start back to running now.
Most half marathon training programs are 12 weeks long, with the longest run at 10-12 miles. There are free ones that you can download, but most of them have you running 4x a week. You can get by with 3x a week as long as you cross-train on at least 2-3 other days with things like cycling, swimming, zumba, etc. I found a really good beginner's plan a few years ago on coolrunning.com, but they no longer have it. I posted it on a blog and you can look at it at www.sparkpeople.com/mypage_public_journal_
. If you have any questions about it or have a hard time figuring out what it says, shoot me a Spark e-mail and I'll translate it for you.
Good luck on your HM goals!
"God gave you your body as a gift, so you should take care of it." - My Mom