Too many hard workouts in a week
Are you the type that exercises at all cost; thinking if you miss a day its going to mean goal not achieved? If you go into most workouts tired, you will limit your ability to push to a level where you can truly get faster and stronger. Plan your training so you give yourself a light/ easy week once every four weeks. You have to listen to your body, if you feel sluggish or weak, take a couple of days to recover before jumping back in to the hard workouts.
Workouts that don’t have a purpose
Most people training will just do whatever they feel like or whatever fits in their schedule with no thought as to what they are trying to accomplish. If every workout does not have a clear purpose (increase strength, build endurance, work on skills, recover, etc.), you are most likely not going to race to your potential.
What others are doing, determines how you train
With training log websites, social media and blogs prevalent, it’s easy to have a 'keeping up with jonesy' mentality. If you have a mate that is training for the same race and you see that they just logged a 20km bike and you haven’t gone further than 10, its easy to ignore your planned workout and get in a forced 20km so you don’t “fall behind”. Having a defined training plan and sticking to it (unless over-trained or sick) will keep you from feeling like you need to compare yourself to others.
You don’t push yourself enough
If you want to get faster or go the distance, you have to put in the work. If you are doing the same type and length of workout in week 10 of your training plan as you were doing in week 3, then you are not unfortunately going to improve. As you get stronger, you will be able to do more intervals or increase weights, or do them faster or with more power.
Tracking data and seeing the gains in your training is a good motivator tool to push yourself harder and keep you on track.
You don’t work on your technique
Getting technique right is vital for improvement in race times and preventing injuries. A few lessons with a swim coach, strength trainer etc can ensure the work your putting in isn't going to go to waste.
I do enough exercise, I can eat what I want attitude
The bottom line is that you cannot exercise enough to overcome a poor diet. Processed sugars and carbohydrates are full of empty calories and nutrient depleting substances that they hurt performance. If you are training your body to be a machine on race day, you cant expect it to perform when you are fueling it with junk! Splurging every once in a while is not going to do any major damage, but don’t think for a second because you train so much that what you eat has no effect on your body or racing performance!