It’s always optimal to train for a race in similar conditions in which you will be running. So if you are running a marathon outside, then it’s best to train outside if you can. Don’t despair if a treadmill is the only way you can train because it’s still possible to do well. You may experience more muscle soreness as you won’t have trained to adjust for running on pavement. Set your treadmill for negative incline and do several runs at minus two percent. This will increase the pounding on your legs that you will surely experience on race day.