Aerobic fitness is probably the single most important factor in determining the success that a distance runner or middle distance runner will have on the track (or on the cross country course).
That said, it is very hard to cultivate aerobic fitness unless some other key ingredients are not being added at the same time. Heavy volume without good form leads to injury. With the occasional session of 8 to 10 x 60-100 meter strides, the body learns what to do in order to run most efficiently.
Once a runner achieves a reasonable level of aerobic fitness and solid running form, the next stages of competitive development fall into place much more easily. Sessions at maximum heart rate or aerobic threshold become more effective and are less likely to lead to injury or burnout.
For beginning competitive runners in high school or at the middle school level, attaining a solid level of fitness is often the greatest obstacle. It takes time to get fit and track and cross country seasons just aren’t long enough. It’s extremely important to train in the off-season!
Here’s an excellent article about different types of fitness.