All, I am looking for some opinions regarding how often to change up marathon training, (introduce new stimulus, new workouts, etc. ) to avoid staleness/ plateauing and adaption to the training to the point where improvement is no longer happening and to avoid regressing. I have used the Hanson's plan, with some minor adaptions (longer long runs and extra mileage during the week) for my last 5 marathon training cycles. But my last two races (Boston 2017 and Boston 2018, missed Portland 2017 with a hamstring injury) have been subpar, mainly due to weather but I didn't feel as prepared/fit as I used to going in. This Boston I felt great at the start but knew I was off by 10 miles. Not sure how much was due to the weather and how much could have been a training issue. Going forward, how often should I change things up (switch programs or make enough changes) to prevent staleness or plateauing?