​If dexterity is how well you can move, and strength is moving other stuff then endurance is your ability to just keep moving. The other stats play their part in Endurance; Strength allows you to move with power and Dexterity aids in graceful, pain free movement. But Endurance is what you get when you add cardiovascular conditioning and muscular stamina. It becomes your inner gas tank that represents both "energy" and "vitality" in fitness and in life. In short, your ability to endure more.
Perhaps the most famous example of endurance is the epic "first Marathon" when an ancient Greek courier named Pheidippedes ran 26 miles to tell of the Greek victory over the Persians at the battle of Marathon. Legend has it that the professional runner took the message from the battlefield all the way to his home town of Athens where, upon delivery of the victorious news, his heart burst and he died on the spot. Today, people do the same thing but instead of  cardiac failure, they get a t-shirt.

After all, that Greek guy DIED FROM JOGGING! Nobody ever looks like they're having fun doing it. Do I really need cardio?
In a word: yes. You do. Smart cardiovascular and endurance training can impact your lifelong health and longevity in powerful ways. Your heart and lungs are the batteries to your body so if they don't function optimally, nothing else will. Since these soft organs are only made stronger by stress then rest (AKA training. see the STR post for stress-response and GAS info), that means that regular bouts of cardio activity are vital to not only your performance and fitness goals, but also your mobility and independence when you hit 80! Extra stamina also means you get to enjoy doing fun things for longer periods of time. When you don't get tired, you can dance and hike, play outside with the dog or just get fun places quicker, all without having to stop for a breather.

There many ways to improve your cardiovascular endurance and muscular stamina. Like strength training, there is a scale of intensity to be aware of when you're working on your stats. 
Whether you choose to go high, medium or low intensity in your workouts, consistency is what really matters. One workout a week (no matter how awesome) is NOT sufficient. Move more, get more!

On the other hand you have shorter work periods with much higher challenge ratings. Most competitive sports, martial arts and "functional" fitness require these intense bursts of activity known as sprints. Training sprints can lead to amazing improvements in weight loss, endurance and overall athleticism, the thing about sprinting is that it's hard.  HIIT (High Intensity Interval Training) is a popular method of training that really works. The Tabata format is a super quick workout that feels like you fit 90 minutes into 4 minutes!

​Unless you have some Boots of Elvenkind hidden in your closet, improving these stats will take time and practice. Lots of practice. I would treat dexterity like a skill in need of tuning rather than as a mountain to climb (like STR can be). Everything from daily mobility care to handstand practice to nailing the water bottle flip requires patient, dedicated repetition to forge neural pathways and condition the body to moving in new ways. If we must start somewhere, I recommend we start with....

​Useful for executing sick backflips, improving performance in any sport and generally getting places faster, speed and agility is a cross between strength and dexterity in training. You need stronger muscles to make your body move faster, so resistance training (specifically plyometric training) will play a big part. Agility is tied closely to balance, but is associated more with movement. Acceleration, deceleration and change of direction should be challenged and practiced to improve your agility.

In my previous post, I defined strength as the "ability to generate and exert force, either explosively or or extended "sets" of time." which is a fancy way of saying moving stuff and yourself through space. What this means for D20 Athletics is that strength is the basis how I train; Whether you want to lose weight, climb a mountain or finally beat your grandma at leg wrestling, getting stronger not only should come first, it must come first. You can get stronger no matter where you start from. Let's look at how bodies adapt for strength and a few types of "strong"​ we can try to achieve.

In order to get strong, you first have to get GAS. General Adaptation Syndrome is a fancy name with a funny acronym for the reactions of your body to patterns of stress. When you challenge your body repeatedly, it automatically finds the most efficient way to adapt to, and overcome, that challenge. 
These adaptations represent a spectrum of strength, but to achieve it you'll generally follow simple steps:

1. Do a hard thing until you get better at it
2. Now make it harder
3. Repeat

​This process is called "progressive overload" and there's a story about a greek guy named Milo of Croton that sums it up perfectly;

Once upon a time in ancient Greece, a young man named Milo wanted to be a famous wrestler and began training with the best gym buddy a guy could ask for; a young calf from his father's flock.
Each day, Milo would hoist the juvenile steer onto his shoulders and carry it up a hill where it could graze before Milo would pick it up again and carry it home. Every day, the calf got a little bit bigger and heaver. And every day, Milo would have to get a little bit stronger to complete the climb to the hilltop. But climb they did, for years until both had grown up.
Thanks to progressive overload, adult Milo could pick up and carry a full grown bull! By training consistently and doing a little more each day, Milo enjoyed a career as an elite athlete that lasted over 20 years!