the last thing you want is bad press. anytime this is not good, but when you are just starting out it can have long lasting consequences. Perhaps you could have them over to “preview” the video together. You should be able to get a read on their reaction. Based on the reaction you could offer them some from of compensation (I suggest you have it already in mind prior to meeting).
Either that or offer a refund or additional free services (i.e. film to DVD xfer, etc) right up front. That is of course assuming that the product they are going to receive deviates significantly from the product they were shown via your demo.
The lenght you described doesn’t “bother me”. I have delievered many videos that were around 30 mintues or less. If you can effectively tell the story of their day in 30 minutes that is all that really matters, after all no one said wedding videos mus be a certain lenght (unless you committed to a lenght of course).