Focussing on an object does not depend on how far away it is, but on the length of the path that the light follows to reach your eye.  If the mirror is flat and not convex or concave, that path length is exactly the same as if the light had travelled as far as the object seemed to be behind the mirror, plus your distance from the mirror. In other words, the effect is the same as it would have been instead of the mirror there had been a pane of clear glass with the object behind it.  The upshot is that the path length of the light is the sum of the distance between you and the spot on the mirror surface behind which you see the image, plus the distance between the object and that spot. This in turn is just what you would have seen if the mirror had been replaced with glass and a (mirror image-shaped) object behind it.

You might have some fun trying to draw the paths on paper to illustrate this.

Cheers,

Jon