If I was the long love letter writing type, I wouldnt expect a similar letter in return.
Me, I'm the long waffley scattered letter writer and I am soooo glad I dont get like letters in return.
I would just avoid anything along the lines of "I concur" or "Me too"
Honestly, girls I know, like to know where they stand and if they can read about it in black & white...all the better.
Just say what you feel comfortable saying, dont jazz it up too much, if thats not the kinda person you are in real life.
Good Luck