Training hard doesn't always mean you are training smart. Intensity is needed to create adaptive response. Indeed our model of fitness stands on the concrete belief that exercise needs to have enough mechanical elements, ROM, and intensity to stimulate neuroendocrine/neuromuscular response. The more the better.
It is important to realize the effectiveness of this model will be dependent on other factors, mainly rest and nutrition, summed as recovery. This is simply a piece of the puzzle that you can't ignore. You must rest well and eat well. Otherwise you are inviting overtraining syndrome. Listen to your body. Learn it and treat it with respect.
Intense exercise will create a positive hormonal environment. However, if that stress is prolonged, that environment may become hostile. Your body is there to work for you but it will only take so much abuse before it protests. One interesting hormone that comes into the picture is Cortisol. We want to keep it in check. This is one reason why our "metcon" workouts are typically short. Metcon workouts should also be dosed. How much you can tolerate per week will depend on many factors but I would suggest no more than 3-4 per week.
We structure our weekly schedule so you are able to get strength/skill work as well as conditioning workouts. Keep track of what you are doing and get some of each. Feel free to do a distance workout on occasion as well. Remember variety is good. Any time you can do something "different", take the opportunity. Don't just train hard, train smart!