That sounds easy enough. But how long should the effort be? I make this decision based on the type of events for which the athlete trains, but for cycling I use 2 to 4 hours of steady AeT exercise as the common range regardless of the event. If your race durations typically fall into the 2-to 4-hour range, simply train for that duration at AeT.
For example, if you do the bike portion of a half Iron-distance triathlon or bike road race with a typical time of around 2.5 hours, then do 2.5-hour AeT bike rides (not including warm-up). Should your race times be less than two hours (criterium bike racing or Olympic-distance triathlon bike portions) then your AeT workout will be two hours. If your event takes longer than four hours (Ironman-distance bike and long road races) your AeT workout will be four hours in duration. AeT workouts of this duration are then done once or twice weekly per sport in the base period. That's all there is to it.
More: How to Balance Your Training Volume
Measuring Progress
Actually, there's a bit more. As with any training you need to know if you're making progress and when you've done enough of such training to consider moving on to the build period. This can be done by comparing heart rate to another metric (power and pace are the best) and measuring cardiac drift relative to that metric. In an aerobically fit athlete cardiac drift will be minimal. Here's how I do this.
On a bike with a power meter, I have an athlete complete an AeT ride and after the ride upload the power meter's heart rate and power file to Cycling Peaks, an excellent analysis software available through TrainingPeaks.com. The AeT portion of the ride is then separated into two halves. For each half, the average power is divided by the average heart rate. The results are then compared by subtracting the first-half quotient from the second-half quotient and dividing by the first-half quotient.
This creates a Pw:HR ratio percentage of change—a measure of cardiac drift (actually, a measure of power changes relative to a steady heart rate that's wanting to rise). When the athlete's Pw:HR shift is less than one percent, I consider AeT fully developed and this goal of the base period accomplished.
More: The Ideal Heart Rate for Ironman Training and Racing
With the development of GPS and accelerometers this same procedure may be used for pace-based sports such as running and cross-country skiing. Simply substitute pace (Pa) for Pw in the procedure above. For runners I use AeT workout durations that are 1 to 2 hours long. (Cycling Peaks software will soon support GPS devices, also.)
Essentially, when an athlete is in good aerobic condition his or her heart rate and power or pace will stay closely coupled at aerobic efforts as described above. If power or pace drops off relative to heart rate, or if heart rate rises relative to steady power (65 percent of CP30 power is a good approximation of AeT power) or pace the athlete is said to be aerobically decoupling. If this is greater than a one percent shift, then more aerobic training is necessary.
Even if you don't have a power meter, GPS or accelerometer you can still do the workouts using your trusty HRM. In this situation you'll have to base decisions about your aerobic endurance fitness strictly on perceived exertion—over time does the effort at AeT seem to be getting easier.
I've found AeT workouts to be one of the most effective ways of training for endurance athletes in the base period. Give it a try and let me know what you think.
More: 5 Reasons to Train With a Power Meter
Improve your power for your next triathlon.