Cross Training is critical to the success of a well rounded exercise or training program.
Our body learns to adapt to physical activities very well. It becomes far more efficient at performing that activity. It uses less energy to accomplish the same task as time moves on.
Well trained runners can not cycle well. Well trained cyclists can not run well. Not unless they are cross training and pursuing both activities. Our heart and muscles respond to a running challenge very differently then a cycling challenge. So a cycling friend who puts in 100 miles per weekend might have trouble running a few miles with me on the weekend.
You will also find that this can significantly drop your risk of overuse injuries from sticking with one particular exercise.