Answer:
Well,there could be an explanation for this. As you start to run, your acceleration begins increasing. This causes your heart rate to rapidly rise within a relatively short period,to enable circulation of blood and oxygen to vital centers such as the brain and muscles. Consequently, your BP and respiratory rate increase,as well as your pulse rate. Since this is happening just within an instant of starting to exercise,the body has to compensate and adapt quickly by burning a lot of calories primarily through glycogenolysis and your muscles,which were hitherto at more rest,now have to contract and relax rapidly, continuously. This causes early fatigue.
On the other hand however,as you keep on exercising,the body adapts to the routine of the exercise,and you are able to keep going. Don't be deceived,you will wear down, but more gradually, because at this stage, your cardinal centers and muscles have become more "adapted" to the routine of the exercise than when you first began the exercise.
Step-by-step explanation:
I hope this helps