My understanding of zones (up to now) had been that Z2 is the Recovery zone, Z3 the Aerobic zone (and best fat-burning zone), Z4 the anaerobic zone (good for pushing your anaerobic threshold up) and Z5 is basically your max. Am I using different zone definitions?
I think of Z1 as Recovery, Z2 as Endurance/Aerobic, Z3 as Tempo, Z4 as Threshold, and Z5 as Supra-Threshold (basically VO2Max and higher). Those are HR-based zones assuming you know your LTHR. FTP-based Power zones are similar except that Z5 gets broken down into additional zones (V02Max, Anaerobix Capacity, and Neuromuscular).
Even though I train with power, I tend to focus on the HR zones for recovery and endurance rides. Usually my Z2 pace will be around 19mph or so. Power zones are most useful for intervals and threshold pacing.