STINSON BEACH, Calif. — The toughest part of the Dipsea, said to be the country’s oldest trail-running race, might not be the first mile, which contains nearly 700 stairs rising through the forests above Mill Valley.

It is not the jiggly-legged drop into Muir Woods, or the steep rise up Dynamite, so named because your churning legs might feel ready to explode. And it is not even the next big hill, called Cardiac. (If your legs have not burst, maybe your heart will.)

It is not the treacherous plunge toward the ocean, the crooked depths and broken steps of Steep Ravine, even the poison oak that crowds the skinny trails and tickles the legs before blooming into a postrace rash. And it’s not the course’s maze of permissible shortcuts, like the one named Suicide, that give locals an advantage, as long as they stay upright and do not get lost.

No, unexpectedly, the toughest part of the 7.5-mile Dipsea, a topographically schizophrenic romp that was first run in 1905, could be mental. It is knowing that the slowest runners are given head starts and the fastest ones begin at the back. It is like unloading a zoo’s worth of animals in reverse order of mobility and releasing the cheetahs at the end.