You probably heard the saying 'A bad plan is better than no plan'. Well, when it comes to training, that definitely tends to be true.
Having a structured routine aimed to improve specific aspects of your athletic performance, even if not perfect, will always trump just randomly exercising when you feel like it.
Whether you want to improve your overall performance or increase your VO2 max, a structured plan will get you there faster.
A structured plan also allows you to have a better mindset, as you can follow the plan and not waste time thinking what you should or shouldn't do.
It also allows you to gauge your progress over a certain period of time, which is not only the most reasonable way to consistently make progress, but it might also increase your desire and motivation to train.
And speaking of training plans, you shouldn't neglect strength exercises. Building up your core strength is a very important, and often very neglected aspect that protects you from injuries and improves your athletic performance in any sport.
Of course, your athletic performance is also related to your health, which brings us to the mistake number three.