When you reach around 35-40 years of age, it can be helpful to modify training to include fewer runs per week but run them at higher intensity. For instance, instead of running 5-6 days per week, try four runs that cover all the “good stuff” like interval or tempo workouts, hilly efforts, and longer endurance runs.
It’s also important to extend your recovery period after workouts and races since the body doesn’t spring back as quickly as it once did.
This might mean adding another rest day during the week. I compare it to taking care an aging car; it will run for you if you provide plenty of TLC