I think we're getting hung up on a combination of symantics and human nature. Nancy was cancelling her Friday morning run. Carrie asked about Saturday and Nancy said she wasn't available until Sunday because she was planning to paint on Saturday. She had not finalized that with Jessica but Nancy wasn't going to schedule yet another run that she would have to cancel because Nancy was planning to paint. If Jessica had told her that she couldn't do it Saturday morning, Nancy could still call Carrie back if she actually even felt like running on Saturday a.m. That's the way I saw it anyway.
That is how the prosecution would like this detail to be viewed, but since Nancy didn't have plans to paint until 12 hours later, it's surprising that the running friend testified that Nancy claimed to have the plan before it was a plan. I think it's also possible that Nancy did not say that she was painting or did not give a time for painting, but the running friend and the painting friend compared notes such that the running friend (who spoke in absolutes) made her story match the other story.