My dirt paths run the perimeter of my town (for the most part). I have a few branches that go down to the river for the campsite, center of town where my park is, and to the waterfall seating. I've planted bushes, avenues of trees, and/or flowers to line them, so it was pretty easy to see where I wanted to walk/run.
Most of the paths are 2 tiles wide, so I can get by a villager, but in some areas, I brought it down to 1 tile, to make it more interesting and give a winding feel to things.
If you're starting them now, know that they will disappear over the winter, from the snow, and you'll have to run the grass down again in the spring, but that might be a good thing, if you decide you want to change course or establish something different. You can get used to a certain track, plot it out, and then when the snow goes, you'll already be in the habit of going that way. 
