No, JROTC is not hard if you are not in good shape.  The class is designed to help you develop leadership and become a better citizen.  The only physical activity is cadet challenge and only happens once a year. During cadet challenge all you have to do is make an attempt at the activities.  Your grade in cadet challenge is not based on how strong you are or how fast you can run.

The only physical demanding thing in JROTC is Raiders, but that is an optional team that a cadet can try out for if they want.