I went 1000 miles at 28 weeks and no problems but feet swelling.
Take regular breaks/ stops if by car and bring pillows/ cushions and alternate them between hips, sitting on them and supporting your back. Even with flying its ok, supportive stockings and getting up to walk about once every hour. Id just be more worried about the boredom of 600 miles! Lol
I have an appointment on Wednesday and my plans to travel are on friday. So ill see what he says. 600 miles away just for my baby shower ! Wow lol
Most ob's are OK with you travelling at this stage. So long as there are no complications. But I would suggest calling the Ob. That way they at least have a heads up. :)