Just hop on and start riding. If it starts feeling more and more comfortable after about 100 miles, it's working. I have never tried either, but leather tends to adapt to the situation (or ***, more specifically) at hand. I don't see any reason why it wouldn't mold to fit you over time.

The hardest part about leather saddles is the initial break-in, when the leather goes from hard as a board to vaguely supple. Since the first owner already did this, all you need to to is get the saddle to custom fit your poterior a bit. It shouldn't be as bad as a fresh break-in, as long as the former owner's pelvic bones weren't shaped totally different than yours.