@croydon - My father used to take me out running and one thing he would always encourage me to do was to run as hard as possible at the end, before the warm-down, basically to use up any energy that was still available. Even if I'm not doing a strict interval training, I try to do that because you get different benefits from different intensities of working out.
Interval training is not just a means of increasing your ability to work hard. If you could just work very hard all the time, with no consequences, it would still be better to do interval training in order to get the benefits of working out at different levels. Your muscles will burn different kinds of energy and gain different kinds of stamina.