We don’t usually say:
When I was young I was going to the park.
The past progressive is normally used when we are comparing two actions, with one action happening during the other:
I met Sally as I was going to the park.
When we want to talk about things that we did in the past but don’t do any more, we can say ‘used to’.
I used to smoke. He used to be really fat. I didn’t use to like olives (but I do now.
We can also use would in the same way, but we must put a time reference:
When I was young, I would go to the park every day.
When I lived in the countryside, I would go hiking on the weekend.
I hope this answers your question. In my experience, French speakers have difficulty with the progressive tenses because they are over-taught in schools. The best way to progress is to move away from grammar based learning and concentrate on lexis, that is, what people really say. ‘Used to’ and ‘would’ are good examples of natural English use.