I don't know how to describe her physical appearance, because she changes a lot during those 50 years. At first, she was a very beautiful woman, and after all those years, she appears as an old woman who still remains a bit of her youth beauty.
The thing I'm sure about are Megan's thoughts: she was like her father, a very confident person, who wanted to have all her future planned. That's why Huw's letters made her feel bad at first.