You do not have to be a world-class athlete to join the military. Even within the ranks of Special Ops, you will not be required to be a master of any element of fitness -- above average maybe, but not world class.
My observations from training many military members over the past two decades has shown me that we all come from different foundations of fitness. We all excel in different events, and suffer weaknesses in others. It takes a mature and ego-free team player to realize that your preparation to be 100 percent ready for your job may be lacking. When you make the decision to go Special Ops, you must be prepared to research your future profession and acknowledge there are elements of fitness you will have to attempt that you may have never been exposed to.
Your best bet is to be competent in as many of the following elements of fitness as possible.
Strength: Being strong and having a foundation of strength is critical to ALL of your other abilities. This does not mean that you have to bench press a truck. It means that having strong muscles, bones, and connective tissues will assist in your ability to make power when you need it. The most basic way to measure strength is to record the amount of weight lifted in one repetition. Don't skip leg day!
Power: You cannot have power without strength and speed. The faster you move an object or yourself through space is power. Power usually requires a full body movement generated from your feet and legs and transferred across the body to its end point. For instance, a powerful knockout punch starts from the feet as the fighter steps into a punch, shifts the hips, torques the torso, and extends the arm until the moment of impact with her or his fist. That is power. In physics, power is defined as power equals force times velocity or work divided by time. It is a combination of technique, speed, and strength.
Endurance: Cardiovascular endurance is necessary for nearly any activity, including running, rucking, and swimming. Technique helps with the amount of energy you use, but being able to move and move fast is one element that has to be continually practiced. If you do not lift for a week, you will typically come back stronger. If you do not run for a week, it feels like you are starting over when you run again. Whether you like fast interval cardio or long, slow distance cardio -- just get it done. You need both depending upon your job. How fast you can run, ruck or swim longer distances will be the typical measure for your endurance ability.
Muscle Stamina: Combine high repetition muscle stamina with endurance and you are building a PT test-taking machine. A two minute calisthenics fitness test is one way to test your muscle stamina, but another marker is putting in a full day of hard physical work. Having the ability to continuously move your body weight and more over longer periods of time is required in the typical selection programs. Strength is handy. You need it. But being able to work all day is a physical skill and mindset that needs to be fostered daily.
Speed: Testing speed with short runs can save your life when having to quickly run for cover. Speed can be enhanced by adding in faster and shorter runs to your running days.
Agility: Accompanied with speed and balance, agility is how quickly you can move from side to side and change direction quickly. Both speed and agility can be practiced with cone drills arranged in less than 10 second drills, where full speed and changes of direction are measured.
Mobility / Flexibility: Do not forget to warmup and stretch for flexibility, but also to move your joints through a full range of motion for mobility. Like many elements of fitness, if you don't use it, you lose it. So make stretching and moving in a full range of motion part of your day.
Hand / Eye Coordination: Whether it is shooting, driving, flying, throwing, or lifting objects to be placed a certain way, having a background with hand eye coordination is helpful to any tactical athlete. Sports can be a great for building this skill, but obtaining good hand / eye coordination requires practice.
Running / Rucking: Being prepared to run and ruck takes time. Time spent logically progressing your weekly mileage in running and building time under the weight with rucking has to be a foundation of your training if attempting most military and any Special Ops training program. Lack of preparation will mean injury and possibly failing to meet the standard within a few months of training. If you don't practice several days a week to build your endurance, you will lose it.
Swimming / Water Confidence Skills: Not having a pool to train in or not being comfortable in the water is not only a physical fitness issue, but a huge mental block for many. Technique is critical to your success in the water. Watch videos and practice, practice, practice if you need to get better in the water for your swimming, drown-proofing, and treading tests. Several days a week of technique training is required, along with building your cardiovascular endurance to maintain any speed.
Specializing in too few of these elements above can lead to neglecting others. World class athletes specialize in only a few of the above for their athletic events. For instance, take the competitive Olympic swimmer or power lifter. Both are incredible to watch, but both would fail miserably at each other's events on an Olympic stage.
The reason I am focusing on comparing world class athletes to those in the military is that far too many regular Joe's attempt workouts and training programs designed for world class athletes. There is no need to try an Olympic swim or running plan used by your favorite Gold Medalist to help you pass a fitness test of a 500m swim or a 1.5 mile timed run -- even if you are trying to be a Special Ops team member. Trying to deadlift 600+ pounds, which is a massive amount but still nowhere near world class, may cause injury or interfere with your ability to run, ruck, or swim with fins for long distances. You need to ask yourself what you have to give up to compete in an Ironman Triathlon, do a body building competition, or power lifting meet. If your answer involves too many other elements of fitness, you may want to reconsider whether this is a necessary step toward a tactical profession.
There is a quote often used in Tactical Fitness Training: A world-class athlete needs to be an A+ in his/her activity, which may only focus on 1-2 elements of fitness. A tactical athlete needs to be a B in ALL the elements of fitness to best do his/her job. Make your annual training plan so that you can arrange the elements of fitness into your year accordingly. Learn about periodization and do it logically, with smart progressions so that you do not start off with too much, too soon, too far, or too fast, and end up hurting yourself with challenging programs designed for something not related to the Tactical profession.