15 hrs/level seems insanely slow to me. I didn't really keep track of how long I spent on each level, but i know 79-80 took me 6 hours. I basically jumped into northrend at 68. The order I followed was Borean Tundra, Howling Fjord, Dragonblight, Grizzly Hills, Ring of Blood in Zul'Drak, Scholozar Basin, and finished up in Icecrown. I only dual box so I didn't really skip many quests on the way and I picked up the achievement for all zones i stuck around in.