Imo, you should get a real tank different shoe. The speed steroid from your q is good enough. Also you should get aegis first in all of your builds. It is a mix of hp and defenses. Hp helps more early game then just pure defense. Maybe you should go like, giant belt->chain vest->negatron cloak->main tank items. Or drop one of the defenses depending on how unbalanced the other team is.

Some clarification on rushing thornmail and FoN:
For some reason I get these really weird matchups pretty consistently where im tanking a team that has 4/5 champs as ad carries or maybe 4 ap carries, and I've found it beneficial to rush either thorn or FoN depending on the situation, but obviously that kind of strategy isn't going to be viable in a game that has even 3 ad or 3 ap, in which case I would go with my base build, starting out with the Legion, then going on with fairly balanced armor/magic resist. I also find that the thing that really makes rushing thornmail or FoN against 4 ad or ap champ teams respectively is just the sheer amount of armor and magic resist each give, for as I say, thornmail doesnt really start destroying ad carries until you get your warmog's. And on the boots, I've tried other alternatives, but as I've stated in the guide, it really just doesn't work, because without boots of switfness, garen simply can't get to places fast enough, whether it be a tower to defend, a fight to tank, or anything.

hmm... Garen has good damage potiental, but i guess he can tank. But really? rushing a thornmail? Unless ALL their carries were either DPS or AD, I would not recommend it. Nor should you rush a FoN. Aegis sounds decent I suppose. And instead of Boots of swiftness, Perhaps get Merc treds? the tenacity helps and you really do need some more Mres. It's not like you'll need to chase anyone down as the tank, thats the anti-carry's job

+0 for now. I think you need some work, but this has potential.