Then you've got the problem that, if you write the letter and tell her not to reply, you've said so much, maybe asked a bunch of questions, but you know that you'll never get a reply to them.
You've got questions now, but how would it feel knowing you have asked them, but will not get a reply?
If i were you, i'd write the letter and start communication up that way.....if you know that you'll never be together, why not stay friends?
And if writing to eachother is an easier way to stay friends, but not get too close, then where's the problem?