Hope, I just wanted to tell you that after 10+ years of interacting with stepfamilies in trouble, this is sooo typical. I'm not sure what the motivation is, but try not to let it get to you. It's all about pushing your buttons. If she got you to react badly, she would confirm to herself that you are bad. Great choice to do nothing until you get your thoughts gathered!
A way to handle this that could work well, in my experience, is wait 24 hours, then say something like 'Just saw your text - had no idea you liked running, that's awesome!' Nothing more, since you don't have time to write at length. And then text her the kids' times next time you take them for a run. That would take the wind completely out of her sails. If she can't push your buttons, she will hopefully stop.
I think not responding at all could seem like she got to you, too.
M 16 yrs, WH62, P54 3 adult blended kids EA 11/13, BD1 6/14 PA fall 14, BD2 2/15 Piecing 2015, BD3 12/15 Separated 4/16 WH moved OW in 5/16 Divorced 6/15/17
And then text her the kids' times next time you take them for a run. That would take the wind completely out of her sails. If she can't push your buttons, she will hopefully stop. 