If what you want is to make the animation of that little leg of the R that is already created as a Shape Layer, I would make a pre-composition and use an animated mask to make it appear so that the little leg is growing little by little.
I would do that in Illustrator using Blend/Expand/Release to layers workflow explained in these tutorials. You can get a lot more predictable results in a lot less time than monkeying with masks, a lot more efficient than trying to animate a path, and completely predictable. Here are a couple of quick tutorials I made a long time ago. I've been using this technique to morph between shapes since you could create blend modes in Illustrator.
You'll need to break off the front leg of the R and probably make two or three drawings progressing to the last frame, but that's the way I would do it.