Unless the 4-5 mile commute you currently do is completely exhausting you, then 12-15 should be no big deal. Just keep riding. Bring some water or whatever, maybe a snack. Starting from scratch is one thing. Starting from riding consistently is another. You don't say how many days a week you commute, but if it is more than one or two, you should be able to ride quite a bit further.

Obviously pace yourself. Don't try to maintain 20+mph the entire time, just ride at a comfortable pace. You should be able to carry on a conversation while riding and not be gasping for air.