Gradually increasing pain during a run with increased effort is how I think of suffering. It should go away when you stop or back off and it shouldn't linger into the next day or two except as sore muscles.

Sharp pain is not usually under the same category. Sometimes it shows up on a run but isn't there on the next run. In this case it's not worth worrying about. But if it is coming on consistently with effort and shooting pain into your legs, you may be causing nerve irritation in your back.

I would recommend backing off the speed, or moving to a softer surface such as trails (Like RnR suggested). If you're already running slowly for that increased distance, I would try going to a 10 min run, 1 min walk schedule when you are going to run long and don't increase your long runs by more that 15 min per week. Then I would add more core strengthening, maybe a pilates class to your training. After about 3 to 4 weeks of this, you could go back to trying some faster running or more continuous effort on long runs and see if the sharp pain is gone. It may be that as you increase your distance, your core musculature is unable to stabilize your low back area and this in turn leads to the sharp pain. Hence the need for abdomenal strengthening.