I mix in some HIIT with some of my after workout cardio. I like having depleted glycogen stores from the resistance training to start using fat as a fuel source right away as opposed to after ~20 minutes. You will get conditioned fairly quickly if you stick to it and won't be crawlin to your truck.
Eating right after resistance training is great but the window is much wider than many preach as far as catabalism and glygogen replenishment are concerned IMO. As long as you are not maintaining the high intensity intervals for too long or hard (hard relating to HR) you will be fine. If you are going really intense for any extended periods a small serving of carbs is all you need between resistance and cardio or sip on them throughout your workout. It will not hurt to throw some protein or BCAA's in there as well but it is not essential.