Need insights about my cardio routine!!

[heehaww]

Hey guys and gals! I am trying to get into the best possible shape, i.e. superior speed, strength, endurance, etc. The cardio arena is where I am most inexperienced. For the last 5 months I have been running a total of 8 miles per week. I usually spend 2 days doing interval workouts. Both interval workouts include a fast-paced jog for a 1/2 mile at a pace of 3:20 and two minutes of rest between intervals. I usually do five of these intervals per workout. So, that puts each interval workout session at approximately 2 1/2 miles. Like I said, I do two of these interval workouts per week and then a standard 3 mile run in 25 minutes on Saturday, which brings the total to 8 miles per week. Lately, I feel like my body is getting used to the intervals so I decided to switch things up a bit. I decided to replace one of my interval days with an agility workout. I feel that sudden explosive power could come in handy for me one day. My question is this: If I do a 30 minute agility workout (cone drills and suicide sprints), will this hurt the endurance I've worked so hard to build these past 5 months? Also, how many calories am I burning during this agility exercise? Thanks in advance for all help and for taking the time to read my post.