Running on the treadmill (exclusively) will hurt your road 5k performance. On the treadmill, because of the belt action, you do not engage the glutes and hamstrings enough to simulate the push-off you exert on the road. You can try to compensate by running at an incline and that will take some trial and error.
The treadmill is better than not running at all on rainy days and such, but you should never train for a road race by running on a treadmill. Unless you are a gifted athlete... very gifted.
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