I decided to take one for the team, use myself as the guinea pig (and Taylor, but he just didn't have a choice), and see how it went. So far, one month into outdoor climbing, I feel like, strangely, my AE actually has gone UP by not training it at all.
Here's the how and why...
Call it what you will, Lactate Threshold, Aerobic Threshold, or Anaerobic Threshold. Regardless, it's the target of many of the Low days in the High/Low Approach. Essentially, this threshold is the point at which your body is producing lactic acid faster than your muscles can remove it. This, we know, makes our already bulging forearms turn into vascular balloons, which causes elbows to resist gravity, which causes us to succumb to gravity. Not a pretty thing. There is a way to raise this threshold, and that is of particular interest to athletes who wish to follow the High/Low system (and it's not for everyone... but more on that later).
Sports research has proven that there are two main ways to raise this threshold. The first way, and most common to climbers, is through interval training. By going back and forth across the threshold, we can steadily raise it. 4x4's are the popular method of interval training, and are an absolutely important tool for climbers, and are not to be discounted. Here, however, we're going to discount them. The reason is that they take far too long to recover completely from, and the intensity of a good 4x4 falls smack dab into "middle intensity". The moves aren't hard enough to be high intensity, but you're causing too much muscle damage to be low intensity.
The second way, which falls squarely into the low intensity zone, is by exercising just below the threshold at a steady rate, and over weeks, gradually increasing the duration. This type of exercise serves to create a more dense network of capillaries, meaning that more oxygen can make its way to your muscles in a faster, more efficient manner. Because these workouts are low intensity, they actually aid in recovery, rather than inhibit it, allowing you to do a session on a "rest" day between high intensity workouts. As far as climbing goes, there are several ways to achieve this.
#1: ARC. Aerobic Restoration and Capillarity. This method of training has become popular over the last several years, and while I'm not a big fan of it, it certainly has it's uses. It comes in handy here, if you can gauge it correctly, and if you have the facility for it. Basically the goal is to stay on the wall for 20-30 minutes at a time, climbing with a warm, but far from debilitating, pump. If you have a treadwall, or the space to set up a lengthly traverse, or a wall all to yourself, ARCing is a valuable tool. I find that for most people, there is a steep learning curve that comes with knowing how to gauge the climbing, and you easily slip into middle intensity training and end up blowing your whole session.
#2: Circuits. My favorite method, by far, is to do a long, easy circuit in the bouldering gym. I move steadily, not in any kind of hurry, and do all of the easiest graded problems in the gym. This can be anywhere from 75-100 moves of V0-V1 climbing. Then I move up a grade, and do around the same number of moves at V2ish. Then V3 and V4. If I'm still feeling good, I'll do many of the V5 and V6 problems that I have totally dialed in. I only take a 5 or 10 minute break after I've completed a grade level, and then I go back to climbing. If I feel pumped even for a second, I back off or quit entirely.
#3: Outdoors. If you live near an outdoor climbing area, and can do your low days outside, then lucky you! Spend these days doing as many pitches as you can get in at least a few letters below your consistent onsight grade, or even harder if you have it dialed.
#4: Routes. Particularly if you are unsure of yourself on ropes, or need to get back into the flow of roped climbing, then this method is for you. Simply get on the wall, preferably indoors so you can switch it up, and lead up and down easy routes, connecting different lines, being creative, whatever. As long as you keep it easy enough to not get stressed, and hard enough to not be restful. If downclimbing grips you, then lower and immediately get back on for a couple of toprope laps, maybe finishing by pulling the rope and doing one final lead up an easy for you route.
All of the above workouts can be done while also working on other aspects of your climbing, be it focusing hard on your footwork, working on stopping that annoying habit you have of regripping a hold 9 times before you move, getting better at heelhooking or other techniques, and learning to climb either slower or faster. It's always best to learn new skills in a low stress situation, and these low intensity workouts are the perfect forum.
Keep in mind these aren't the only low intensity workouts in this program. Far from it. These are merely to answer the question of "Well how the hell are you training endurance and power at the same time?". The answer is, we aren't training endurance at all. We're training a bunch of other things, and our "rest" days just happen to be increasing our endurance.