With foot corns that are super old (in this case, 20 years) or super deep, I like to REALLY get the dead hard skin out.
I also make a point of levelling out the hard skin ring of callus that surrounds them. This alone can cause a lot of pain.
(I use Gehwol Fusskraft Mint to soften this and make it easier to scrape off)
With foot corns this big and established, they often benefit from another treatment about a fortnight later to settle them down.