A Couch to 5K plan is definitely the way to go. These programs work through progressively increasing intervals of running and walking. Typically they are around 8 weeks, but if you are already running a bit, perhaps go to week 2, which should put you right on target. www.sparkpeople.com/resource/fitness_artic
15 min/mile (4 mph) sounds about right for mostly walking with a little bit of running. But at this stage, you shouldn't be too worried about speed. Your focus at this stage should be on the motion of running, rather than your speed. Keep your pace down at this stage.
Also, running is hard on your body, so beginner runners should run just 3 times per week to allow your leg muscles time to recover and strengthen.