I am nearly half way through my first eight week course, even though we have dabbled in the past, and it is all done on-lead. Why not do a course and see if both yourself and your dog naturally likes it. I would be wary that when doing agility the dog gets a lot more excited than normal obedience and therefore may get slightly more aggressive when all hyped up. Some dogs when doing their agility course bark a lot which may also get her hyped up. Why not try the eight week course, even if you feel safer using a muzzle and see how it goes