Don't sweat Molten Core and some of those larger zones for now. It'd be very surprising if there are guilds still running the zone; unlike 5-mans, Raid Zones are tiered in such a way that they aren't meant to be done once new expansions come out.
For becoming a better tank... You don't need to respec Protection unless you really want to tank. You have to love what you do, or at least love the idea of becoming better, to spec Protection before you are level 68 or so. Heavy tanking for most people is done at the endgame, or, the current level cap depending on the expansion.
We have some very, very helpful people on the forums. I'm hoping they can respond to you a bit more in-depth than I can at the moment.