Assuming you have a high level boosting you, the moment you hit 56 head to Outlands and start farming Ramparts. With RAF you'll go back to 15-20 minute levels pluse you'll have a good amount of TBC gear which is obviously far superior to any non-epic gear pre-TBC.

At 40 I did BRD (though I think ZF is faster to 45). At level 52 I switced to LBRS until 56. I actually had a great time exploring BRD and LBRS. I hadn't really been through the whole instance in a couple of years and I was shocked at how relatively difficult and massive those instances are comparted to TBC and WotLK. I miss the days of CC, screwing up and wiping, etc. If you pull two groups in WotLK then just AoE more... try that in pre-TBC and it's a wipe.