From the point of view of sports science, doing just aerobic exercises doesn't really make sense.
[spoiler]I would assume that just doing aerobic exercises are somewhat slow, and if load isn't gradually increased, the body gets used to the stimulus given and does not develop any more, from the point of view of conditioning and sports. I would assume this also would have an effect on the energy cost of that exercise, and especially recovery. There's three ways of making pure aerobic exercises more "efficient": Increase distance, increase speed and decrease breaks. Naturally, you can do aerobic intervals, as well, and vary the composition of the exercise. And then the important one would be adding some alactic anaerobics (sprints) there.
To keep things simple, as a major division, the fibers in the muscles you can control are divided into two broad categories: Fast twitch fibers and slow twitch fibers. Aerobic exercise, typically, mainly uses slow twitch fibers. Adding short (non-lactic acid producing) sprints (under ten seconds) allows the body to use the fast twitch fibers, as well, while producing minimal amounts of (performance-degrading) pyruvate. This increases the amount of energy your muscles can use (mitochondrial growth) faster than just using one muscle type. Naturally, if the subject of the exercise has never done any fast exercises, the "fast" cells will be somewhat underdeveloped in the muscle tissue. I don't have any specific solutions to that except for "keep trying" - they will develop later in age, too, though the preferred solution is "do your sprints before your puberty is over". Considering EVE's demographics, it's likely pretty late for that solution to be practical.[/spoiler]
Naturally, for sports, there's a whole world on what you can do with intervals. On a whole lot of things, the high intensity intervals (actual effect depending on how the exercise matches your strength, repeats, weights/zone, etc.) may not be ideal either. (An increase in anaerobic power has the tendency to decrease both anaerobic and aerobic capacity, i.e. endurance and lactic acid tolerance.) This, of course, assumes again regular training (>3 exercise units per week).