Don't wait, start right away. You can't really devote more time on it, than your 10 games per week, so it's limited to the number of weeks you played.
5v5 gives the most points. You can also be the most useful there with a prot spec. I personally went for a heal/sta mixed gear and cast some heals at the start, to clearly reveal I'm a healer. When I got the attention, I resort to instant spells as much as I can, so I can't get interrupted, which basically means I'm trying to cleanse every single dot the team's getting, while the other healer (you should have 2 healers in your 5v5, especially if you're prot) tosses around the actual heals.
It's not perfect, but your rating won't drop too low if you play your balls right.
I enchanted mine with +40 Spelldamage. Together with T4 Shoulders and +20 Spelldamage on my gloves that puts me on 311 Spelldamage. With Spirit buff, Adept's and Oil I'm around 400 spelldamage. I already have a hard time keeping aggro with that against the DPS in the raid (Progress: 5 bosses down in SSC (all but Vashj), and Void Reaver and Al'ar down in the Eye). When I'm on the first mob to be nuked, I switch to more Tier gear for 600-700ish spelldamage.
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Zoe's guide to building your tankadin