Fitness and health are not the same. Neither are activity and diet. It's not possible to outrun any physical disorder of any kind whatsoever, including obesity. To illustrate, consider health as the basis for fitness, or the potential for fitness. The higher this potential, the greater the benefits of activity, but no amount of activity can compensate for a lack of this potential.

Think of Pottenger's cats and generational epigenetics. With subsequent generations eating a equally bad diet, health gets worse and worse until a point of infertility. For any particular generation, it's possible to begin restoring health back to optimal over the next few generations by using an adequate diet, but it's impossible to do it for the current generation. It's even less possible to do that with activity alone.

Now consider two individuals with less-than-perfect health. They have the same fitness potential, but neither have the best possible potential. Now put one on an exercise regimen, while we keep the other on a bench. The one doing some activity will appear to improve his health, but only because his health is the potential for fitness, and his fitness has improved compared to the guy on the bench, but only so much as his health allowed.

Now consider two individuals, one with perfect health, the other not. Put both on an exercise regimen, the one with perfect health has the potential to reach the best possible fitness, while the other has a lesser potential. The point is that the one with less-than-perfect health cannot possibly reach the same fitness level as the guy with perfect health, because his potential - health - is less.

We can see this with sports teams and some individuals adopting a low-carb diet as a way to improve their game performance, and it works quite well, but now imagine if we started this with their parents, and their parents, and so forth, until we finally restored perfect health for the individual a few generations down the line. His health will be that much greater, so his fitness potential - therefore his game performance potential derived from the same amount and quality of practice - will also be that much greater.

Oh yeah, forgot. Fitness is not just physical ability to perform, it's also neurological ability to perform. This is due to the fact that strength for example is neuro-muscular in nature, so the act of practice, even if muscles don't grow bigger, produces greater strength and the ability to lift heavier weight for example. In my case for example, I play golf, and I have an extensive amount of practice in my bag, so while I'm not physically fit, I am certainly able to perform the required motions with a much greater degree of precision compared to somebody without that practice in his bag.

So, with practice and study, we develop, improve and maintain skill. This means fitness is skill, or at least in large part skill. This is pertinent for fitness where we need time, endurance and something called strength-endurance to practice enough to improve skill, to improve fitness for any particular activity. The very act of practice requires health for an adequate amount of practice to be done, so the act of practice cannot in and of itself compensate - well, it can but not fully - for any lack of health. It can compensate for a little bit, because with practice, we improve skill, you see?