Some sentences --- corrections are welcome!
Could you check these to make sure they are correct? Please & thank you

I wanted to change the rules. Quería cambiar las reglas.
Last week, I ran in the marathon. La semana pasada [no comma] corrí en el maratón.
Tomorrow, I will go to her house. Mañana [no comma] iré a su casa.
I could not believe she said that to him.
No podía creer que se lo dijo a él.
I will have to discuss that with her at the meeting tomorrow. Tendré que discutir eso con ella a la reunión mañana.
2 Answers
I wanted to change the rules. Quería cambiar las reglas.
"Quise" is also possible. It depends on what you want to say. English does not care much about the distinction, so it is hard to explain it.
Last week, I ran in the marathon. La semana pasada, corrí en el maratón.
Good, but in Spanish the comma is not used unless you deliberately want to make a pause there.
Tomorrow, I will go to her house. Mañana, iré a su casa.
Same thing with the comma. A pause after "Mañana" sounds a rather weird here. It is almost like saying "Tomorow. I will go to her house". That comma should be removed.
I could not believe she said that to him. No podía creer que le hablaba a él.
This one is not right: "... que le dijera eso (a él)". Once, more, both "podía" and "pude" are possible... depending on how you want to present the facts.
I will have to discuss that with her at the meeting tomorrow. Tendré discutir eso con ella a la reunión mañana.
"Tendré que..." and "...en la reunión".
No podía creer que eso se lo dijo.
Tendré que discutir eso con ella mañana en la junta.
The rest are great!