Not sure I can help but were the times that far out with the factory settings, before you calibrated?
Mine was wrong because it was recording kms, even though set to miles. This was a known fault corrected by a software update though. It sounds like yours is more wrong than that. There's a forum for this sort of thing which might help, either via the Nike or Apple website, not sure which. Might be worth doing a search unless someone on here comes up with an answer.
Hope this is of some use at least.