I was involved in a project recently where we tested the fitness of the traveling teams for a fairly large club. The boys and girls were committed players, 12 - 18 years of age. We tested speed, anaerobic capacity, power, agility and endurance.
In most cases, the results improved with age because many tests are related to muscle mass that increases with growth. However, one factor that is pretty sensitive to training is endurance.
Unfortunately, the results from the endurance test were not as predictable. For the girls, the average endurance values were essentially unchanged from age 12 to 18.
The 12-year-olds had the same endurance as the 18-year-olds? Yes, they did. For boys, there was a bit of an improvement at around age 14, but was little changed after that.
You might guess that the 18-year-old on a traveling team could be hoping to play some quality soccer in college. From that pool come some players for the U18 national team, who run twice as far or more on the same test. So that 18-year-old has a long way to go to catch up to their age national team.
Yes, the national team trains harder, longer and more frequently than a youth traveling team. But the troubling factor was little or no change in endurance over the middle and high school years. Coaches seem to be focusing training on technique and tactics and neglecting the endurance component of fitness.
Remember that a strong fitness component needs to be a part of each week, three times, for about a third of each practice (competitive games count).
So, assuming one game a week, two sessions need to have physically challenging periods of training. Compete twice a week, and one more session is needed.
Improving endurance is easy: Go out and run. But the endurance for soccer is trained best within the nature of the game that is a series of short higher-intensity intermittent runs with periods of rest or reduced activity between runs. Thus, endurance can be improved with interval exercises.
Track and field coaches have been using interval training for decades and their experience and research shows that for short runs, a work to rest ratio of 1:3 is pretty good (e.g. 10 seconds' work, 30 seconds' rest).
When one looks closely at the game, that is about what happens. Players run at a pretty high intensity for five to 10 seconds, then rest about 30 seconds before running again. You can design off-season training around this concept repeat runs (faster than a jog, slower than a sprint) of 10 to 15 seconds with 30 to 45 seconds rest.
It sounds like the best way to improve endurance is to just play the game. But we know that a training 11v11 game is played at a pretty low intensity, so the way to make the game more intense is to reduce the number of players, which leads to more ball contact, the most intense part of the game.
To make the game faster with shorter sprints and shorter recovery periods, reduce the size of the field so the game is more compact, forcing quicker reactions, decisions and running. To make the game more demanding in terms of running volume, increase the size of the field.
Inserting an endurance component is not difficult, but does involve some planning. Consider this game: Move the goals (carefully!) in from the end line so the field is around 60 yards long. Face the goals away from each other. A low-intensity 6v6 game is free play. A moderate-intensity game is to limit midfield (between the goals) passes to three per possession, forcing players to play faster.
Finally, play a game to six points, but if a team can score with all their team in front of the goal and leave at least one of the opponents in the midfield, they get two points. Same game, but adding restrictions increases the pace of play. You would need to plan the amount of time on each game so that around 1/3 of the practice is of the highest intensity.
You can take almost any basic game and increase the intensity by adding restrictions such as two-touch, run 10 yards after each pass, all up in the attack. Restrictions can be combined, like two-touch, then run 10 yards after each pass.
The idea is to insert restrictions on play to keep increasing intensity that will improve endurance. If all practice activities focus on skills and tactics, then the endurance is neglected and your team will tire late in the game, when lots of goals are scored. Improve your endurance, and your team will be the one doing the scoring.