Fitness isn't simply about being able to do a push up or having a good squat. Fitness is how well your body performs daily at work, rest, play, and sleep. Basically fitness is every bodily function summed up in a word. So, for lack of a better analogy, pooping properly is part of being fit, and being fit requires hydration, as does every other function.
We can see the effects of exercise on our bodies, the slimmer waist, larger biceps, longer cardio times, and these are good things. What we can't see and where bad things often begin is with our internal performance such as digestion, hydration, cellular reproduction. Proper nutrition is paramount to excellent fitness because a machine can not operate to full capacity if one of it's systems are compromised, just as a living body can't.
One of the most common issues (40% in adults) that causes severe medical issues for older adults is not being properly hydrated. Being properly hydrated is a shield against urinary tract infections, cognitive impairment (slowed thought process), low blood pressure, increased tendon and ligament issues, poor skin, slow digestion, and more. If you think just drinking more water is answer, you are only partially correct. Your nutrition also dictates how well your body retains and uses fluid. Medications and excessive sugar will also wreak havoc on body's hydration.
Your nutrition is just as important to being properly hydrated as is drinking water. Having foods high in minerals, such as vegetables and organ meats is highly important to remaining hydrated. Hydration isn't simply more water, it is also proper nutrition at the smallest level to ensure the water gets where it needs to go. Taking mineral supplements is recommended due to modern un-regenerative industrial farming models. This is becoming more and more evident Remember, the USRDAs are the bare minimum to maintain bodily function, so adjust accordingly. Start with the big 3 - calcium, magnesium, and zinc. Always get blood work done to track nutrition and supplement progress, making you more pro-active in your health. From there, determine with blood tests and by general feel how they work and their dosages.