You shouldn't place anything over the areas where you want your paths, though. If there's a design or whatever over the top, it will stop the grass from becoming degraded. But @ Freckles is right, a good way to ensure you're running across the exact same area every time is to line your paths with things. This will cause your paths to form sooner. Also, avoid excessive time travelling forwards as this will cause patches of your grass to turn into dirt and probably ruin your paths (as far as I know, time travelling backwards doesn't cause this, but I'm not 100% sure).