OP, I strongly encourage you to ignore all advice regarding stem length here and go get a proper fitting done - preferably one from a trained fitter and not just the free one from your LBS.

A proper fitting will ensure that not only are you comfortable on your bike, but that you are maximizing your power. Also, it will save you a lot of time, energy and money in fiddling around with different stems.

Lastly, I also would advise you to not do huge rides after making substantial changes to your fit. This is a recipe for injury.