YOU SAID:

I went the park to walk then I had to go to the bathroom

INTO JAPANESE

私は公園に歩いて行ってからトイレに行かなければならなかった

BACK INTO ENGLISH

I had to go to the toilet after walking to the park

INTO JAPANESE

公園に歩いてからトイレに行かなければならなかった

BACK INTO ENGLISH

I had to go to the toilet after walking to the park