I started at 68 on 2 toons this weekend. I use Jame's horde leveling guide addon. I finished Howling Fjord to the point I got the zone achievement and only skipped 1 quest due to level and 1 quest (the sleeping king) due to the queen not being up.

I was over 70 when entering Borean Tundra so no leveling issues. I am 85 quests in and have 75 left for the zone achievement.

It has gone much faster then I though. I have the heirloom items, but the heirloom cold weather flying at 68 helped far more then anything else.