It's about variety.
Ayres has competed in everything from one-milers to ultras of various distances. In the sixties, he was obsessed with marathons and hoped to make the U.S. Olympic team before eventually discovering that he was better suited to be an ultrarunner. (The best marathoners, he says, are not pure endurance runners but also have great leg speed, which wasn't his strength.) Still, his experiences training for and running different types of races have led him to take a wide view of the concept of cross-training.
That may include methods such as one favored by Scandinavian athletes, called fartlek (irregular shifts of speed in order to surprise the body), as well as allowing both "associative" (highly focused) and "dissociative" (mental wandering) running. He also tells the cautionary tale of a runner who trained for a mountain ultra by running on a steep incline on a treadmill – without accounting for the course's quad-killing, mile-long descents. Whatever the method, Ayers says that by combining complementary types of training, runners gain the benefit of adaptability, durability, and resistance to injury.