Running outside vs running on a treadmill vs running in the same spot all have differences. (fitbodyhq.com)
I'm a terrible runner so my preferred cardio is brisk walking but I live in a very cold climate and walking on a treadmill for long is not so much fun. (rachaelattard.com)
Start with 25 minutes on the treadmill at a slow, relaxed pace, just enough to get your heart rate up. (health.com)