Garen has two forms of burst, his Q and his R, and the only one he leads with under normal circumstances will be his Q. Much of his damage comes from his E which is sustained damage, and if you're rooted/stunned for most of the duration it's a serious drop in damage output.

Garen already has a nice MS steroid in his Q and will often build a FoN. He's doing fine for speed. Tenacity doesn't fit anywhere else into his build. Merc Treads is definitely ideal unless you're playing against a low CC team.