When training for a road race, you need to practice on the road. Don’t do all your training on the treadmill. You have to get out there and experience the pavement, the changes in level ground and elevation. Often we train on the same route or track, but in the event you may have to go up and down curbs, around corners, over dirt paths, concrete and asphalt. This can all effect your running form and if you haven’t practiced on it, may cause your shoe to rub a different way and lead to blisters.
When road running, trail running, or the like you are using your core more, trying to maintain balance while your land on uneven ground. Training on a treadmill is always the same softer, easy landing. Also, a treadmill pulls your leg back when you land and when you run on ground you have to propel yourself forward, not relying on the treadmill to pull your leg back for you. You need to be able to push yourself forward and not try to keep up with the pace of the treadmill.
If you will be running a hilly course, make sure you practice on some hills. If its mainly flat, get used to running at the same pace and exertion over an extended period of time. The nice thing about a hill, it gives your body to chance to switch it up and use your muscles in a different way while you climb the hill, then you get a little rest as you coast down the hill.