It's wrong. Why? A 'pal' is a friend. 'Penpal' is a person, not an activity. You can't say 'I did friend', can you? So you can't say 'I did penpal'. It makes no sense. You can say 'I had a penpal' or 'I wrote to a penpal'.

It sounds a bit odd,maybe rephrase it to : You had a penpal for such a long time.