15 minutes is significant. Is that EACH way? In the morning, I'd rather have that 15 minutes of extra sleep, and I do NOT want to talk to anyone in the morning. In the evening, I just want to get home, get dinner, and put my feet up ASAP and I do not want to talk to anyone other than family (or others of my own choosing). Sounds massively inconvenient to me.
So true, how DOES she get to work when you are sick or on vacation? Is there no public transportation? Surely you are not the difference between her being able to get to work or not.
I have difficulty saying no too, especially once a pattern has been established. I'd be full of excuses. Lots of great suggestions here, but I like the idea of the sandwich technique. with a little white lie as the filling: "You're a great passenger, very considerate, but it won't be possible for me to carpool with you anymore. My situation has changed and I won't always be going straight home after work. I really don't want anyone relying on me for a ride. You are a great passenger though, so I am sure you'll be able to find other arrangements."
The only problem I foresee is if she asks to continue in the mornings only. In which case you can say it's no longer working for you; you are unable to leave early to collect her, your family needs you, etc etc etc. But I agree, don't use the gym as your excuse or you'll find you have yourself a new "workout partner." (Wasn't there another thread a while back about someone who wanted to work out alone at the gym after work to unwind, and a coworker declared herself as a workout partner and followed her there every day?)