When I started running, my easy pace was 11:00/mile and my heart rate was 145-150.
Now my easy pace is 8:00-8:30/mi and 125-135 HR and I have to run at ~7:00/mi pace to get my HR into the mid 140s.
That took well over 2 years and the improvement wasn't all that noticible run to run, or week to week, or even really month to month.
But the improvement was very clear after 3 months, then 6 months, 12 months, 2 years, etc.
Problem is, most people rarely give any program enough time to work.
Very few people consistently follow a plan for even 3 months, let alone a year.
But if you do, you won't believe the results.