I wouldn't think so. I would let them know what your plans are and phrase it, "I thank you for your kind offer/invitation for me to drive with you; however, I have gone ahead and booked a flight," and give them the details. You might want to do it via email if you can initially, so that they have the information in writing in front of them. Also, it doesn't pressure them to feel as though they've been put on the spot, etc., in case they are upset. Do it as soon as you can, too, in case they want to change their minds and fly.
Since she is beign such a wonderful hostess, maybe you could arrange to take them out for a nice meal? I know a lot of times, people won't take cash, but purchasing a dinner (and giving the CM a heads up or taking care of the bill while you've excused yourself to a restroom) is always a wonderful gesture. Be sure you write her a thank you note after the trip and let her know just how wonderful a time you had. I made DFi's mom a scrap book from the trip we took together (and she paid nearly in full for). It was one of the Disney scrapbooks with the year on it. I put in the pictures in and gave them all captions... some sentimental, some funny, some just the location. She treasures it.
You could always make them a CD for the drive down.
I think if you go about it so that they know you are thankful, there will not be a problem. If you don't, it might come off as rude.
