Meg used to hate PE classes.
When I was younger, I used to visit my grandparents every week. = When I was younger, I would visit my grandparents every week.
Used to можно заменить на Past Simple без изменения смысла, когда речь идет о привычных действиях в прошлом или о состояниях:
I used to watch cartoons. – I watched cartoons.
I used to have a dog. – I had a dog.