You can ask for leave for military training, but your situation is that you usually exercise too little, and normal doctors can't give you a certificate. No certificate is required for military training on leave. You can try to follow the training first, and don't like to take time off. Military training is not only to improve one's willpower, but also to get along with new students. If you don't participate in military training, you will lose an opportunity to get to know each other.