I would suggest you begin using a training program for new runners (See link below).
What I always tell my new runners, while having a goal time in mind, until you have done the training and have an idea how well your body adapts to the sport of running, it is best to put that on hold for now. Once you do your first race post-training then you can start setting your sights on your time.
Remember too, that you cannot rush the process in becoming a runner. Allowing recovery time between runs is essential since it is when you are not running that your body makes the adaptation to running.
I hope this helps!
RUN SPARK STRONG!
Coach Nancy www.sparkpeople.com/myspark/training-plans