You did the right thing here, OP. The woman is entitled.
Next time you feel yourself getting nervous about this situation, think about how her time mooching has taken time away from your children. Even if they were in school during the rides, chatting with her took time that you might have been thinking of things to do with them, or even just thinking happy thoughts about your kids.
Work up some [deserved] righteous indignation about how she has been using you, and ignore any attempts to make you feel guilty.