Jeremy and others:
When we wrote the TftNA book we really debated using the Zones concept. The reason was that Zones as determined by a % of max HR or any other arbitrary % measure are just that….arbitrary. They probably get you in the ball park of determining intensity but no guarantees. In the end we gave in to pressure to include the zone idea because it is so commonly referred to popular literature on endurance training. It is the one area of the book I would like to re write. But if you read carefully we did give you a handy way to determining the top of your aerobic zone using ventilation markers (these are an actual reflection of your real time metabolic state) and there is considerable writing here on UA concerning that in our resources section.
The point of finding the upper limit of your aerobic zone (Top of Z2) is that this is the metabolic limit of your aerobic system to produce energy from fats. The best way to improve this, the aerobic capacity is to train at and just under your aerobic threshold (AeT) so that you are maximally taxing your aerobic capacity while minimally involving your anaerobic metabolism.
If you must go very slowly to do this then all this is showing is that your aerobic capacity is very low and has a lot of upside potential yet for development. Training above the AeT in hopes of raising it is a fools errand. Above the AeT your metabolism will have shifted to predominately anaerobic glycolysis and you will have exceeded your aerobic capacity. The dominate training effect above AeT is to the glycolytic metabolic engine NOT the aerobic engine.
So while this may feel ridiculously slow if you have ADS (Aerobic Deficiency Syndrome) that is all the more reason you need to be training in Z1-2. For folks with ADS this will be very low intensity. The best stimulus for the aerobic system is duration, time on your feet in Z1-2. As you get fitter and your aerobic capacity increases you will be moving faster at your AeT and you AeT HR will increase as well. For very fit endurance athletes AeT pace is FAST training and they can not handle very much of it. Usually 1-2 AeT workouts/week is plenty for elite athletes. It is too muscularly demanding for those with a high aerobic capacity. But for those with ADS they can do EVERY workout right at their AeT because from a muscular stand point the work level is very low.
Here is a real world example to help demonstrate (and quantify) the aerobic capacity AeT concept:
The running marathon is an event that EVERY runner competes are his or her personal aerobic threshold. It doesn’t matter if you are running the 26.2 miles in 2 hours and 5 minutes (2:05) or 4 hours. Provided that you are motivated to push as hard as you can for the full 42km you will be bumping right up against your personal AeT. For a world call marathoner this AeT pace means running faster than 5min/mile (3min/km) and the slow folks are running 10min/mile (6min/km) at their AeT. Obviously it takes a lot more muscle power to run 3min/km than it does to jog along at 6min/km. So, while the world class and the weekend runner are both utilizing there full aerobic capacity, the world class runner is producing over twice as much power with his muscles for the same metabolic loading. The world class runner can’t train every day with long runs at his marathon race pace because it is so muscularly demanding. Where as the average recreational runner can run ever workout at marathon race pace. I know most readers here are not marathoners but these same principles apply to climbing mountains. A big aerobic capacity allow you to move very fast at a low metabolic stress.
I hope this helps.