The problem with Nike+ is that it relies on the shoepod to be accurately calibrated more or less before each run. This is a major problem especially if you complete different types of workouts at different speeds, so your feet may strike the ground more or less often, affecting the outcome.
I ran a half marathon a few years ago with Nike+ and it thought I'd run an extra mile despite me keeping a very good racing line to avoid increasing the distance.
Delete the run and contact @NikeSupport via Twitter with the details you have (distance actually run, time it took, the device you used etc) and they'll manually add the run for you. You won't have any of the GPS info unfortunately but this way, you don't lose out on mileage if you like the achievements on there for example.