I totally agree with Toots!
I know it's hard to say it and at times, it feels quite rude, but you must value yourself first and foremost.
I would have an issue with the commitment. I just don't want such a huge responsibility on my shoulders.
A couple of years ago, my SIL asked me if I could take my nephew to school and pick him up everyday. (my kids didn't go to his school) TBH, yes, I could've done had I wanted to but I just plain didn't want to. (and I love my nephew)
I told her "I can do it for a month until you make other arrangements." She was a bit put off by it but in the end, I had to value my time and energy before her need for childcare. Yes, it's selfish but sometimes, it's okay to be so.
And I'm so glad I didn't have that responsibility on my shoulders for the entire year. I gave her an idea of what I was willing to do (one month) but beyond that, I just didn't want to commit.
And you know, when you stop being the "go to" person for people, they magically find and/or make other arrangements without any trouble. Imagine that!
My SIL was able to make arrangements with her job to go in an hour later and she found a friend's mom who picked him up from school at the same time she picked up her son. Worked better for everyone.
Good luck and yes, you can do it! Just tell her "this isn't working for me. You'll have to make other arrangements." If she asks 'why' it isn't working for you, you don't have to explain. Just say "it just isn't. Please make other arrangements starting the first of the month." And repeat as necessary.
ETA: Although you don't need to explain yourself, I'd probably let her know that you had no intention of giving her a ride this long. That you just need to reclaim your travel time for yourself. It's not rude to put yourself first. Really, it isn't. Took me years to learn that fact but I did and you will too.