pork tenderloin is a splendid cut. it is the same muscle as beef filet mignon - but obviously from pork.... and it is the usual and customary suspect for "most tenderess" chunk - second, btw, is flat iron....
I have twice tried the 'pre-marinated' pork tenderloins (it was a on-sale thing....) and both times were disasters. apparently our tastes in marinates and flavoring does not fit within the mass market appeal.
hopefully yours will be better - I'd sear it "all around" in a hot pan then finish in a slow oven - 265'F - takes about 15-20 minutes from fridge temp - use a thermometer - don't cook past 130/135'F internal.
Ok, since it was free. Its probably just me, but I would never purchase an already marinated/flavored meat product. That question of "What are they trying to hide?", just pops in my head like a red flag.
Recently I got a Hormel tenderloin and steaked it out into big chops. I pan fried one like midwest tenderloin sandwiches and the other night fried some for the pork chop casserole.