My first thought was also some form of cross training. Training methods haven't reached their end form, and will continue to evolve. We know from other sports like cycling, skiing, and speed skating that athletes continue to benefit from a high volume of easy training, so they train in the 25-30+ hour per week range vs runners in the less than 15 hour range. See the thread last month on the unique training of the Olympic speed skater that trained huge hours on the bike and then set world records speed skating.
Already, some runners do add easy training hours by adding lower impact activities. Rupp is famous for adding hours by running on the lower-impact Alter-G. In really old LR threads (when Japanese women were winning Olympic marathons and medaling in the World Championships frequently), people frequently noticed that Japanese runners training in Boulder were doing a lot of extra hours walking. Allie Ostrander always got into great shape coming out of injury training mostly on a stationary bike or elliptical.
You could argue that the best runners in the world don't cross train, but that would likely be because their top competition isn't doing it - yet. In the future, it may (or may not) prove to be a necessary step to keep up with the competition.
Just speculating, but I think walking, maybe particularly up steep hills/mountains, could potentially be a great form of cross training.