I would suggest keeping your training plan the way it is. I'm coach at Div 1 University and our typically long run ranges from 12-18 miles, depending on the individual and the week's mileage. We also do a Threshold Pace workout at least once a week. Your Threshold Pace is probably the most important pace for you to train at for any distance race. Typically this pace will range from current 10k race pace to current half-marathon race pace. The idea behind training at Threshold Pace is that this is the fastest pace you can run without producing lactic acid (obviously if you run long enough you will produce some, but it should be minimal). The more you run at this pace, the better your Threshold will be, which is the basically idea behind training. The longer you can run without producing lactic acid the better you are going to run. If you push your long run, then you will probably start producing lactic acid simply because the distance you are covering is so long. I would suggest to keep any kind of running that produces lactic acid to a minimum. Keep your long run relatively easy. They should be anywhere from marathon pace to 1-minute per mile slower than marathon pace. If you can run 12-15 miles at this pace with ease, I would suggest to start increasing the distance of your long run.