Well, I guess I don't understand the 2H & Ret gear unless you are re-speccing. If you are re-speccing to Ret, then obviously you want the 2H and Ret gear.
If you are Holy spec, then 2H + Ret gear = less DPS. 1H + shield actually hits for more damage and kills faster and is more mana efficient and keeps you alive longer and.......ok, too many "and"s....but all are true.
Holy spec puts out a ton of spell damage DPS. 2H + Ret gear = white damage. Not a good mix.
Now, for your gear - as long as you have a full plate Holy set, then you should be ok in that. Since you are pre-Kara geared, then you may have some non-plate. If so, ditch the non-plate items for plate items. You want armor to mitigate damage as much as possible.
As Tinyknight mentioned, the Living Side wil have runners. You can still use the 1H + shield. Just make sure to use your Judgement of Justice to hold them in place when their health is low. Then, stun when they are about to heal. With those tactics, you won't have to worry about runners. The mobs are high enough level where your main targets will die much faster (as opposed to the ones being hit by consecration), so you don't have to worry about 6 mobs running at once. Of course, that is assuming that you take on one group at a time.
Other than that, you should have a blast. It's a fun challenge. I did it as Holy spec in pre-Kara gear. It took a while, but was a lot of fun. Just be very careful with your pulls. Getting adds are pretty much the only thing you have to worry about.