Are all fit people healthy?
Not necessarily. The line between being super fit and unhealthy is very fine. If one exercises moderately 3 or 4 times a week then one will be healthier than the guy with a gut who watches TV whilst sucking on a beer bottle.
The fine line comes into play when one steps up an exercise program. Being fit is pleasurable, taking fitness to extremes is more pleasurable still and highly addictive. Getting to peak fitness and falling over it into a sort of hell where one is continually assailed by rhino viruses, sore throats and a runny bum, is surprisingly easy.
When one does extreme exercise adrenalin and cortisone are produced, the harder one goes the more of these flight or fight chemicals flood the body. Chems which are very useful in going faster and faster on a running track or a bicycle. As a downside they switch off the bodies immune systems.
If a squillion years ago a sabre tooth tiger chased our ancestor and his immune system didn't switch off, and allow him to concentrate 100% of his energies into running away, then he would not have survived to be our ancestor- who by not allowing the immune system to stop wasting energy on fighting minor viruses allowed him to be eaten.
So fitness is good, extreme fitness not so good. The reason most athletes in endurance are retired by age 36 is not weak legs but a weakened immune system. So the Zen like : less is more, is good advice.