Bill, most thru-hikers (AT and otherwise) will probably tell you that hiking poles don't really build muscle, per se, although some seem to believe they help to maintain muscle tone in the arms that might otherwise be lost over the course of weeks and months of minimal upper body use. In the long run, though, the "muscle argument" for poles tends to fall pretty far down the list of benefits and/or excuses for why experienced l-d hikers use them, among those who do. You might try to eliminate any such bias favoring the use of poles for your upcoming hike in order to best evaluate the immediate merits - if any - on your training walks.
Do thru-hikers "build a lot of leg muscle"? In fact, it's certainly not a given that they do. Personal physiology, hiking style, packweight, and other variables lead to quite a variance among hikers on this. The Appalachian Trail is certainly steep and relentless in its grades, which favors muscle development more than on other long trails, but over the long haul the same effort required to build muscle can also break it down, due to an accumulating caloric deficit, ie weight loss, that often forces the body to break down muscle in the absence of appreciable body fat. Most finishing thru-hikers, especially the guys, look very lean to gaunt overall, toned and wiry in the legs.