I've found that the time it takes to level 5 chars isn't that much longer than levelling a single char...

At your level, questing is probably the easiest way, since you don't get much exp grinding on normal mobs. There are plenty of quests in Silverpine & The Barrens that should get you going...

Once you hit 15 you can start instancing as well, and the exp becomes much better grinding on the elites, not to mention the drops are better as well.

I know it's hard starting from scratch once you've had a few 70 toons to play with, but stick with it, it gets a lot more fun from level 20 or so.

Cheers,
Stealthy