All I do for my paths is create designs and then place them on the ground. I've got a pic of this, which I'm using as my signature. This one is of my cobblestone pattern (and, yeah, that's me standing over a peach so it looks like my butt... ignore that part for a sec). Each path I design has 4 designs worked into it... two main blocks, one left edge and right edge. Whenever I feel like switching out my paths, I go to the able sisters and pull a different path design out of my design storage. I have several rock paths, a couple of tile paths, and a wood boardwalk.
Laying out these designs took FOREVER... and as you can see hasn't really done a whole lot yet to promote grass growth.
It goes where you place it. If doing a straight line as I did in my paths, you could do 2 at once. It would go on the one you're standing on, and the one in front of your character... Then you move up 2 spaces, and do it again. And do this until it goes where you want. Takes a long time to do it so it looks just right. I think the path in my town looks just right =)
or if you are talking about just the regular paths that happen when you run around what yoku do is just run around a bunch and then after a while of running in teh same place then the grass will start to fade