One, this adaptation is not unlimited. You cannot keep going hard and expect to continually get stronger. There are serious limitations to what one can take and if you stress yourself too much and too often, you will suffer long term performance loss. In general, athletes tend to go too hard and think they are invincible, while sedentary folks are too afraid to work hard and properly stress their bodies.
The second caveat is that recovery takes time. You need to give yourself that time to recover. When you are growing up or when you are getting older, you need more recovery time between hard workouts. Recovery is also an individual trait. Some people recover better and faster than others. There isn't much you can do about this and forcing the issue will produce the opposite of what you want. Strict adherence to a cookie-cutter schedules sold on the web or in a book will not produce the desired results.
Everyone eventually needs a long break too. All cyclists in the Tour de France suffer from decreased performance in week three. That despite the weekly rest days. And while some fare better than others, all are in need of some extended rest near the end.
As with training, recovery is very discipline specific. Triathletes may recover from running by going for a swim. Often training plans include yoga or pilates, or weight lifting and stretching, or some other non-endurance, relaxation-enhancing activity. The main reason why this is inserted is because most athletes need more than one rest day a week during their peak training, but most would go bonkers if they had to sit around for more than one day.
Whatever you do, please allow sufficient time to recover from hard workouts. Always make sure you are well rested before starting a major workout, otherwise you won't be able to push hard and derive value from it. And even when you aim for year-round fitness, take a week off every once in a while.