Outside! I only run treadmill for interval sprints. I have a delightful little device called a Garmin Forerunner 301 that is a GPS system and heart rate monitor built together and fits on my wrist. It tells me pace, distance, heart rate, calories, even altitude for when I trail run... you name it, you got it. It's expensive but well worth it. I love to trail run, so to me this is like having the world's best treadmill...