I thought, too, that genetics only controls for something akin to a few percent of your potential, and that to bump up against that limit you would have to already be in the 99th percentile for a particular capability. So your endurance is controlled by past training almost exclusively.

Twin Studies are often more feasible (and sometimes more ethical) than a random control study. A Twin Studies can have results that qualify as "interesting" even with just a dozen or two groups of twins; a random control study on the same sample size would be discounted for its small p value. And with a Twin Study, you can sometimes do post-hoc analysis on years of effect, without waiting the years for those to occur.

This study may even be one of the ones that's unethical to do in a random fashion. We know that lack of exercise is bad for your health; telling people not to exercise for a year could be seen as doing active harm to study participants.

Of course, Twin Studies suffer from some issues on their own. I would prefer random-control tests where they're possible. But sometimes Twin Studies are all that you're going to get. And sometimes the Twin Study of an effect is the economic small-scale pilot that generates interest and funding for the "real" study, and there's nothing wrong with that.