The aim of training is to work on your fitness; to maintain it, improve it, sharpen it, broaden it, or whatever. The problem is, the fitter you are, the harder it is to improve your fitness in any respect. There are strategies and tactics you can use, and your approach certainly needs to change as you get older. But even as your performance declines with age, you can still work on your fitness...if only to slow the slow-down. There are always things you work on, or fitness challenges you can under take - just look at YouTube for people who did, eg, 100 push-ups or pull-ups or burpees, etc, a day for a month!!
The main point is that fitness is cumulative. The training you do today, and that you recover from and adapt to, prepares you for the training you will do tomorrow / next week / next month, etc. They say you need to be fit enough to start training for a marathon (or any other distance), and that even if you think you were as fit as possible for a marathon, this fitness is just a building block for the next time you start a training campaign. Each campaign may differ slightly in the sessions you do, but all with aim of building cumulative fitness.
The most important thing about cumulative fitness, like experience, is that you can't fast-track it. It takes time and patience. And reflection on previous fitness campaigns to help guide your approach to use the strategies that will best help you next time.
The partner in crime to fitness is fatigue. The more you train relative to your fitness level, the more fatigued you will become. And it's not just the fatigue from a single session, it is the cumulative fatigue over time that will build, and build...and build. This is where it becomes a problem. Fatigue is OK in small or manageable quantities, but cumulative fatigue is not. It can be a deadweight around your performance in sport...and day-to-day life.
So, think of fitness as a building block, and fatigue like a Pacman chasing after you. Make sure your building blocks keep ahead of Pacman.