I think there’s too much information for one sentence, and other than the full stop at the end there’s no punctuation.
I would say admired instead of looked up to because it’s less ambiguous.
The phrase when we were young seems tacked on and would be better elsewhere.
When I was young I had a friend called x.
I admired her for her determination, she had won lots of medals as a gymnast and was very popular.
The first sentence may not be correct because I don’t know the relationship between you and her, she may have been your friend, relative, acquaintance or just someone you knew.
The second one is alright though.