What actually does being fit mean? If you watch the Tour de France long enough you'll hear references like watts and VO2max which really are a reference to describe how hard we can go. To keep it simple, humans have 2 basic systems that when trained can give us enhanced fitness. The first one being the aerobic system which is our body’s ability to utilize oxygen as efficiently as possible. The term VO2 max is used in conjunction with this system. Everyone has a genetic predetermined VO2max number or ceiling. Off the couch VO2max is trainable but once at your genetic limit that's basically it, from there at most it's 10% trainable. That ceiling is more or less what you acquired from your parents genetically.
The second system is your Lactate system which is part of your metabolic system or referred to as ones Lactate threshold. LT is the biggest indicator of fitness and the most trainable. It's really a measure of your sustainable power. During light to moderate exercise, concentration of lactate in the blood remains low. The body is able to absorb lactate faster than the muscle cells are producing it. As intensity increases, there comes a point at which lactate removal cannot keep up with the rate of lactate production. This point is referred to "lactate threshold". This is the beginning of the end of high intensity exercise. You can't maintain your pace very long at these levels. The other side is that through proper training you can raise your LT to much higher levels. As an athlete your VO2 max and max heart rate can drop with age but you can still become faster by training your LT properly. An average cyclist might have a LT of around 75% of their VO2max heart rate but with correct training can work its way up to 95 - 98%. That alone shows you how trainable it really is. Your LT is trainable to the point it can keep elevating over years whereas VO2max hits a ceiling pretty quickly. The final desired result is to raise your LT and increase power or watts at LT.