All you had to do was take your photo ID, the letter/card attached, and your medal to any ticket booth. Once verified, the card was activated. They did not verify any name or finish time via their computers. All they did was check your ID to the name on the letter.
A friend who was already an AP holder, got his card activated. He then handed my other friend his card. She walked through the turnstile, where she was fingerprinted for the free day. They did not ask to see ID or medal at the gate turnstile.
Yes, it would have been possible to sell it to someone, but it was a nice of my friend to say here use this, to my other friend. I know of a few other non-runners who got into the parks using the same method.
Another friend who bought the Premier WDW/DL annual pass on Sunday, asked if he could apply the free day towards his new dual coast AP. He was told no, even if he was buying it on Marathon Monday. It had no cash value other than getting one person into one park that day. He ended up not redeeming it to give or sell to anyone because the ticket line was too long in the afternoon.