Boo to both the car and to the race. Major bummer that you spend your money on a new car and it keeps having issues. Hopefully they get it fixed before you have to go the Lemon Law route. (Now I'm thinking of 30 Rock.)
As for the race, as you know, bad races happen. Especially on roads, which are boring, so you have more time to let a bad race get in your head. See how your miles feel by the end of the month before bagging the marathon, maybe try another race (Newburyport 10 miler? Bonus that it doesn't kill a weekend.) and see how that goes. And that vdot equivalents to a 3:33, so if you had a bad day on a hot, humid day, imagine a good day on a cool, dry one.