Starter still running with the key off - If you're certain it was the starter running, than more than likely the starting solenoid as mentioned. But, there's also some aspects of good news.
First, the starting solenoid is on the left side just ahead of the battery, not integral to the starter motor. Pull off the long left side panel and you'll see it nestled in there. Just follow the large red battery cable. Kind of a pain to get to. You'll also want to check the Pre-Starting relay R2, up in the front fuse box just to make sure everything is OK with that.
Also, the starter motor will disengage itself from the engine crank shaft once the engine starts. It doesn't use a bendix drive like we're all familiar with. Instead, there is a Sprague clutch - what some call an over-running clutch - in that gear train assembly. So, the point is that when the engine started, your starter motor should not have been forced to spin at any extreme rate where it might have tried to self destruct.
Good luck with it.
