Totally agree with ennay. The long run is the foundation of distance running training, but if you can't maintain shorter runs during the week as well, then you are not ready. A Rule of thumb is to increase your weekly distance by no more than 10% per week. Typically, a training program will have you maintain your shorter runs as they are and add one mile per week to your long run, up to 10 miles or so for your race distance.
There are a number of beginner half-marathon training schedules on runnersworld.com that may help you structure your running program. To run longer, you need to run slower or incorporate run/walk intervals. Personally, I'm a big advocate of run/walk intervals ... there's tons of people who run full marathons that way. Jeff Galloway is the guru of this approach, his web site is
www.jeffgalloway.com. My advice is to work on distance first, speed comes later once you've built up your running base. For your first event, simply finishing should be your goal.
It's not an unreasonable goal IMO, if your goal is simply to do the training and complete the distance by May. I find that setting these kind of goals help me stay on track, it's no longer exercise, I'm "training". People of all sizes, shapes and ages complete these events successfully, and it is a tremendous feeling of accomplishment to do so.
It's very important to have properly-fitted shoes -- a specialty running store can help with that. And keep us posted on your progress!!