If you are a cyclist who thinks that worrying about heart rate and V02 max is a really lame way to train and thinks that it takes all the fun out of the simple joy of riding your bike, you are not alone. Many other people feel the same way and there are ways you can just go out and ride your bike and get fit at the same time! It will take some planning but you will not need any structured training.
The first thing you can do is go out and meet new people to ride with. If you are riding alone you are probably riding at your natural pace. The problem is that to get in shape you need to ride at all sorts of speeds. By riding with other people you will be force to ride at a faster or slower rate than you normally ride at. The ideal mix to get in shape would be that Five percent of your riding would be all out sprinting while about Fifteen percent at a pace you can just barely comfortably speak at. Then the overwhelming majority of the riding should be done at a moderate to easy rate of pedaling. To do this ratio you should head out riding by yourself or with friends most days, but once a week ride in a fast group ride or with someone who can drop you easily on every climb or sprint. This will force you to ride much faster than you normally would without having to have a specific plan.
When you train with specific goals in mind in terms of heart rate and exertion, then you pay attention to how hard you are working. Since most of the thought during that time will be thinking about how hard you are working, it will actually probably slow you down since you will be concentrating on that effort. If you allow your thoughts to be whatever they want to be while riding you will feel like you are working less and will ride faster and better as a result. If you do everything you can to think about anything other than how much pain you are in during a sprint, your brains relatively limited channels of attention will be distracted with the other thoughts.
Finally, use a simple plan to get your core tuned up. When your core muscles are not strong, you lose your form when you are exhausted during a race which wastes energy and can be dangerous to you in terms of injury that can result. All it takes is three minutes every morning to do three different plank poses. The first is to prop yourself up by your forearms while laying face down forming a plank. The second and third are alternates of each other. Do one minute on your left forearm with only your left foot touching. Then do the same with the right side. This should only take three minutes and by maintaining your body as a rigid board in each of these excersises you will get your core muscles ripped quickly.