Stair tread advice needed

[Pan]

I'm in the process of stripping my staircase down, there is a dark stain on the stair treads, risers. The risers and sides I'm not too bothered about as they will be painted. The main issue is the treads, in the middle of then is a shadowy mark in the middle from where the carpet runner was, I've tried sanding it out thoroughly with no joy. Any suggestions how to get rid of it? Or will I just have to go with a darker varnish in the hope it doesn't show.
Thanks in advance

 

[vinn]

The carpet runner would not have been "in the middle" of a tread.
Anyway, if you have a belt sander then a rough paper will easily remove a few more mm's - dont go any deeper.
If not then live with it - most blemishes "disappear" with familiarity.