If you've ever engaged in major physical excercise after having been inactive for a longer period of time you will undoubtably remember the discomfort that follows. 
For horses it's the same, occasional and extreme efforts are exhausting. A good physical condition can only be developed through the hand of a well planned training program.
The experience I gained whilst training people in my fitness courses adapts perfectly to horses. The training has to be based around the horses physical ability and a goal that is being set.
Modern technology enables us to measure and register heartrate and more specifically heart rate variability in fine detail.
Proper interpretation of that data allows us to set up schedules where constructive aerobic training is conducted.