<font face="Verdana, Arial, Helvetica, sans-serif" size="2">In fact, I think that initially no amount of miles in the world could get you a free F ticket.</font>
Wasn't miles but segments that scored me my first free Int'l F ticket. Some time back in the late 80's, DL had a one time promotion that offered a freebie Int'l F ticket if you flew X number of segments in a short period. Just guessing but I believe it was 32 segments in 60 days.
A friend and I qualified and booked freebies to Athens.
The next year, DL offered a similar promotion except this time it was 24 segments in X days and the reward was a free Business Class ticket anywhere. We booked a couple of flights to Hong Kong in the last days of DL's service there. Never saw a similar promotion again.