Wanderer's Minuet for 30s, Mage's Ballad for 30s, and Army's Paeon for 20s forms your core Song loop. AP is cut off early because it's the weakest of the three Songs while WM is the strongest, and all three Songs have the same cooldown of 80s so WM will be off cooldown before AP actually ends. As for a general leveling guide there is this one by Cetonis. At the bottom of that guide you'll also find the link to the level 80 BRD guide.