You don’t have to train for a marathon by only running outside—marathon training on a treadmill is just as effective as outdoor running. A treadmill allows you to control your speed and keeps track of running distance and heart rate. Unlike hard cement, the running belt is softer than concrete and has a lower impact on your legs and knees, so you’re less likely to injure yourself. Plus, the flat, even surface is easier to run across compared to rough terrain.