I would start right now and tell her that as of: (give her at least thirty days), you will not be able to drive her on: (pick two or three days of your choosing), as you have other commitments and will not be able to go out of your way.
You do not need to tell her what those commitments are. Just say, "Suzie, I am sorry. But, I said I have other commitments on Mondays and Thursdays and will not be able to drive you to or from work. You will need to make other arrangements.
And keep repeating if necessary. If anyone else wants to comment and stick their nose in this business, I would reply, "Well, since you seem so concerned, I will let Suzie know that you are willing to take over the days that I am not able to". Bet that shuts them up because they may say all they want to, I notice they have stepped up to the plate, now have they?