I seriously think I'm having flashbacks here.
I had a friend I used to communicate with via letters and cassette tapes, too! And they weren't those amateur 60-minute cassette tapes, either-- they were the high falutin', this-is-serious-business, 45-minutes-on-each side tapes.
Seriously.
Looking back, I have NO idea what two 15-year-old girls could have POSSIBLY had to talk about for 90 minutes for each exchange, but somehow we totally worked it like a boss!