In that respect, running is no different. Without following some sort of training plan, it is unlikely even the most gifted runner will be able to complete formidable distances. A steady buildup phase involving a mix of distances and pace make up a sensible training plan. It only makes sense that we would want to prepare as best we can for an event.
Runners are by nature overachievers.
Our sensibilities tell us to run in training as much as possible to be ready for a race. The answer to this question of course is not universal. The amount of training that will produce maximum training effect for a runner varies widely among individuals. Age, sex, recent racing and training history, other life commitments and stresses, and most importantly genetics all vary from individual to individual. We all have heard stories of amazing feats by runners.