Well, when you queue for the level 30-40 Wolves Den as a level <50 you a small amount of experience, rank up your abilities in the same way you would normally, and earn wolf marks, which'll eventually be used at level 50. You're right that there's nothing to use wolf marks for prior to that though. In the 30/40 tiers if you're wearing gears above the level of that arena they'll be synced down the the equivalent of a normal quality gear about 2 levels less (needs confirming) than the arena, so if a guy in full ilvl 90 PvP gears goes into level 30 PvP, he'll be wearing the equivalent of ilvl 28 in there. For this reason if you're wearing gear appropriate to that level then you have an advantage over them. Unfortunately healers don't get many of the tools they need to survive until 40+, and after obtaining a few PvP skills, which requires you to PvP.