It sounds to me like you have answered your own question. Choose a better location. Try creating your own 600k route and go ride it. See how it goes. Make sure to make it an interesting route that has the services.
Has the 600's you've failed on also had more meters per km of climbing then the ones you have succeeded at? You could also not be ready for the additional toughness of the extra climbing on the shorter ride. It sounds crazy but it could be something to also think about.