I am aware of many times Charles improved athletes VO2max WITHOUT what he considered "cardio" (running, tredmills, etc), by simply keeping the pace of the workout fast. Short rest times, longer TUT, etc. A better VO2max would definitely help decrease someones run time. Having trained to run an 18 minute 5k in the military, if you are strictly looking to improve your running time then invest in someone looking at your running mechanics. You'd be surprised what you can gain there.