I practice dry-firing in the mirror at the end of the hall about 3X a week. Like you said, it gives you a lifesize image to "shoot" at.
I think you may be confused about the distances, though. If the mirror is, say, 10 yards from where you are standing, then the reflection is that same distance, not doubled. ie, an image of you in a mirror at 10 yds should be the same size as a real person at 10 yds. Not a big deal, but I just didn't want you having to adjust your sights to twice the distance!