I'm afraid none of the flatlanders I've met have been able to do sustained climbs without actually, you know, practicing sustained climbs. Repeats on overpasses just don't do it. Powering up a short rise and then recovering, no matter how many times you repeat, is quite different from finding a rhythm and level of output you can sustain for an hour or more and knowing how to conserve your strength for the long haul. Then there's also acclimation to altitude as the air gets thinner. There's little you can do to train for that short of sleeping in a hypobaric chamber/tent.