Human beings are hardwired to become efficient at the things we do consistently. Artists perfect their paint strokes over time. Endurance athletes are exactly the same. The more we do our sport, the more efficient we become at doing it. But there’s a downside to getting more and more efficient!
The snag is that as you get more fit, if you continue with the same training, the effect in the future in terms of overall gains from your workouts will become less and less.
Think of it this way; in the beginning of the season when you are out of shape, even a week or two of training will net some very noticeable improvements in speed, endurance, and overall efficiency of motion. But by the end of the season when you are extremely fit, those gains become almost imperceptible- even with a huge amount of work. And in terms of metabolic efficiency, in the beginning of the season it takes a large amount of calories per hour to fuel your training. At the end, that number can be a third to half of what it took earlier that same year.
The efficiency gained from consistent regular training is one of the things you want to have happen, especially if you are targeting a long race like a marathon or an IRONMAN. You want your body to be able to go the distance on fewer calories per hour. And building that efficiency only comes by doing the same thing over and over and over. This is your consistency. As an example, the bulk of your aerobic fitness will come from the long endurance workouts completed each week and from the faster speed sessions that, for many people, are also a weekly part of training. All other days that you include are going to be ones that provide smaller gains in efficiency. This is where variety becomes essential.