Conventional wisdom says to keep changing your workouts. It says that if you do the same workout week-after-week, your body will adapt and stop improving. In contrast, I believe doing similar workouts allows you to accurately compare workouts from one week to the next, and from one year to the next. It gives you a way to check-in and see how you are improving. You don’t have to run the same workout for every week, but by frequently repeating the same workout, you can see how your training is improving and make the necessary adjustments.
For example, over the past 2 weeks, I’ve used an 8 mile time trial to see how much I’ve been improving. I was able to improve my pace by 11 seconds from one week to the next.