Soccer, like most sports, is seasonal. There are periods of preparation (preseason), competition (in-season), and recovery (offseason).
Preseason and in-season training typically are the domain of the coach, but the offseason is largely the player's responsibility.
What you do in the offseason can impact the next season. The old coaching adage that "it is easier to stay in shape than it is to get in shape" is true, but most players don't know how to maintain their fitness without a coach supervising them.
You would be correct in guessing that there is a lot of research on gaining fitness, but you might also be surprised that there has been a great deal of study into losing fitness (detraining).
The first real work on detraining studied responses to bed rest and later used people who were recovering from heart attacks, surgery or immobilization. Currently, there is a lot of work on detraining as directed toward zero gravity and space travel.
Training leads to two major adaptations in the body. First is the ability of the cardiovascular system to deliver oxygen to the cells and the second is the ability of the muscle cells to use the delivered oxygen.
Research shows us is that the central cardiovascular system's ability to deliver oxygen to the muscles improves slowly while the muscle cells improve their ability to use the delivered oxygen pretty quickly.
When training is stopped, the muscle cells lose most of what they have gained fairly fast (10 days to two weeks is about right), but the cardiovascular system detrains slowly.
You may have experienced this when you work out after being off for a short break. That first workout doesn't feel too bad. During that workout, the cardiovascular system sort of takes up the slack from the cells that detrained so quickly. However, if you lay off for a month or more, you are starting back at ground zero in terms of endurance fitness.
Now, the question arises as to what can be done to maintain fitness what is the least one can do and still keep most of their fitness?
While you may not have thought too much about it, you know that training is a mixture of three factors: training frequency (days/week), training intensity and training duration (minutes/day).