I leveled with friends in dungeons. I was the tank, they were my DPS. That said, it took us about two months to hit 60 as at the end I worked on leveling my blacksmithing and we all started to independently level around the 56 mark. Depends on what you want to play and how much time you put in. A hunter or mage can level fairly quickly if you're not making a ton of mistakes and on a hunter you can solo a lot of group quests. Which classic has a ton of them.