Your brisket was undercooked, but I doubt it needed 10.5 hours to get done. Don't worry so much about time...every piece of meat will be different, and they all get done in their own time. The 1.5 hour per pound rule is only a rough guide. Most of us will take a brisket to an internal temp (IT) of around 195-200*, then begin to periodically test it for tenderness. The best way to do that is with a toothpick, a bamboo skewer, or a meat probe...poke the meat in several places. When the toothpick (or whatever you're using) slides in and out easily, as through butter, the brisket is done! Your final IT will likely be anywhere from around 195* to 210*. Again, remember that every piece of meat is different, so there's no definitive final temp. When it gets tender, its done.
Hope that helps...Good luck with the next one!
Red