First and foremost there is the notion of "being in shape." This is usually understood as having the ability to tackle anything that is physically demanding without undue stress. People will say, he or she is in shape, I am in shape, and they may assume that therefore, I can work hard, run an ultramarathon, cycle a double century, or finish an ironman.
The problem with this concept is twofold. One is the general nature, the implication that one has the ability to do any kind of work, be it physical construction labor, cycling, swimming, boxing, etc. The general part is quite problematic because we are all bodies that adapt in a highly specific manner to certain environmental conditions and stresses. For most fitness parameters far reaching adaptations can be had and may be needed for success. Without specific training, you will not display these adaptations, and while you may think you are "in shape" you may not be ready for a particular activity.
The second problem is that, while most people equate being in shape with being in cardiovascular shape, others think of it as being trim and muscular. These are two very different states. The first one is far better suited for endurance activities, but even when you are in good cardiovascular shape you should not engage in any endurance activity without specific training. As for the body-building types, with few exceptions, these individuals are very ill prepared for endurance races.
While there are some common cardiovascular fitness elements in all continuous strenuous activities, every activity, be it cycling, running, canoeing, has some highly distinctive components and these components matter a lot.
It is also typical to think that the specificity is limited to the neuromuscular system. I.e. you need leg muscle for cycling and upper body muscle for swimming. While that is certainly true, the differences don't stop there. Every activity produces changes throughout the body, and these adaptations affect every organ system and tissue. Ignoring these facts leads to failure and injuries.
You may be able to run an ultramarathon easily, but that does not mean you will race a double-century, and vice-versa. As a matter of fact, the more fit you are the more likely you are to get injured when you attempt to try something like this.
Lance Amstrong is a great cyclist and he is certainly in great cardio-vascular shape. Yet he suffered miserably when he attempted his first NY marathon. That is not because running is harder than cycling. It is because Lance was ill-prepared and not adapted to running distance. So it should be no surprise that he found his marathon harder than anything he had ever done before. Or that he got injured and suffered shin splints, a very painful condition.
The same rules apply to everyone. Don't go racing an endurance event without specific preparation. Being "in shape" is a meaningless concept unless you specify what it is you are in shape for.