Have you ever heard this great training analogy?
Training for distance running is very similar to moving a very large and heavy piece of furniture, by yourself, across a large carpeted room. The piece of furniture represents our fitness level/performance barrier. Where the furniture sits now is our starting point fitness, and the other side of the room represents our goal fitness (goal time). Now because the piece of furniture is large and very heavy we can’t get it to the other side of the room effectively just by pushing on one spot. We have to push it from various different angles, to get the best leverage and keep it moving in the direction we want, repeating angles and spots as necessary as we see where the resistance lies, to fit it in the desired end location. Each angle and spot we push on represents a different type of workout (LT, groove, VO2 Max, AT, repetition, long run). If we push too much or too long in one spot, we run out of leverage, so we have to change up our spot/angle.