As others have said, it's totally by luck. Either a child is born with a liking for running, or an event happens that turns them into a runner.
I've had several runners in the same tribe in the past, so there's no limit on the number of runners. In one tribe I had five all at once. Two were born runners, one had an event happen to her that turned her into a runner, then I had an event that made two clones of one of the original runners appear.
I expect others will come along to report even more runners in the same tribe.
