Peel it until only the white shows.

Fifty-three recipes here:
Celeriac

Woody- Sometimes I bring home bad vegetables. Woody sweet potatoes, butternut squash, etc., that look fine (to me) on the outside but have hard or woody patches on the inside. You may have got one of these baddies.
Celeriac is not to my liking, so little experience with it. I have used it chopped up in a stew.