Every sport has different types of fitness, cycling is an endurance sport with limited extremely high intensity movements, but nonetheless a very tough sport to compete in...especially goin up mountains!!!
I play basketball & lots of it, i have a mate who plays Aussie Rules, a sport where huge distances of ground are covered regularly in their games, he is very good and extremely fit! ...or at least is on the footy field, he swaps to bball, 15mins in has to have a break, cant take it anymore, game is too fast, this guy is an elite athlete! Basketball, apparently from reading studies is the sport with the most regularly occuring high intensity movements and consequently whilst endurance is a very necessary component (i actually have won countless long distance running events, never training, just because of my bball fitness) its more the acceleration component which drains a person's energy. A basketballer would hardly be able to jump straight across into cycling and expect to win regularly nor would a cyclist fair well the first time he played high level basketball.
If you're talking about football as in American Football, then i presume it is similar to Baseball, extremely high intensity movements, but with large rest periods. Tennis i havent played enough of to comment, but damn that sport looks tough, i certainly admire the guys who go 5sets and still play with fire and intensity
soccer i love it, tiring, but long periods of low intensity movement reduces the necessity for extremely high levels of fitness.
So in conclusion each sport is very different as in "how fit" you need to be, but without doubt professional cycling is right in the very top echelon of sports fitness along with things like marathons, ironman comp's and other similar sports, however a transition to another sport which requires "less" fitness might not be too successful either.
My Q. Are the low-end Trek Bikes overpriced?