Agree with burnman, with the addition that you race like you train. Yeah, you need to train fast-twitch and slow-twitch muscle fibers (there's also an issue with how many muscle fibers within the muscle you recruit with each contraction; you recruit more of them with maximal intensity exercise), just like you need to use a weight routine to build up the contractile force of individual muscles. But in the long run, what's more important, weight training or a tempo run?

You can do a ton of weight training (or long, slow/hard, fast), but if you don't train the way you race, you'll never reach your true competitive potential. So I would argue that you need hard, fast/long, steady (not slow, necessarily; steady)/race pace. Gotta do those tempo/race pace workouts if you want to race.

That would do your body a hell of a lot of good! Too bad its not possible...

The point is that if you do every workout with medium intensity, you don't increase fitness much. By making your easy days truly easy, you are better rested for the hard days that really count and you can properly benefit from those workouts.

It's not so much about training muscle fibers.

Long slow workouts allow you to train your aerobic capacity adn build it up to the maxime level. If you do this at too fast a pace, youre using your muscles a bit too much, robbing them of energy and rest. The rest and energy would be better off being conserved for your faster and harder workouts, where you need it the most.