You are the people who protect/defend/safeguard our country from foreign enemies. You have to be alert/ready to act, at any time when they are given orders. You have to be on duty for the more than 24 hours, which includes both physical and mental strain. One needs much more than a good physique and the determination to spend long, strenuous hours in extent. But unless you are fit and active, you will not be able to give the best output to serve your country.