Last year I kept feeling like I wasn't absorbing my workouts, even though I was trying to be cautious in volume and timing, and eventually I came to the conclusion that I wasn't "strong" enough for the work I was doing. I'm using "strong" in a broad sense here, like not enough base mileage but also not feeling mechanically sound or springy. I had 2-3 years of significantly reduced training (intensity-wise) for a lingering injury from 2015 to 2018 (ages 41-43+), and that put me in a way bigger hole than I realized.
At the beginning of this year, I experimented and revamped my schedule. Early on in the season, I focused on strength/drill workouts while increasing my mileage, with the increase in mileage coming primarily from very gradual increases in my normal easy distance runs, as opposed to big increases in my long runs and workout volumes. As I got into the meat of the season, I also switched to alternating weeks, where one week I would do a "normal" track workout on Wednesday and long run on Saturday, and the next week I would do a low-volume speed/drill/plyo workout on Tuesday, a very light aerobic tempo on Thursday (typically 10 or so miles with 2-3 miles at MP effort - no watch), and long run on Saturday. I was also taking Sundays almost completely off (15-20 minute slow jog to loosen up before doing prehab work). After about 10 weeks of that, I felt the strongest I had felt in years. No single thing I was doing was particularly impressive, but I worked my way up to running about 60 miles in singles, mostly in 6 days, while still getting a great variety of work. I was also being careful not to push myself to run faster or higher volume in workouts.
Unfortunately, there were no races so I never got to see how well that translated to race times.
Someone else was talking about training more steadily throughout the year with fewer significant breaks. That's huge. The older you get, the longer it takes to come back from any kind of interruption. That means you have to consider eliminating blocks of high intensity training followed by time off, in favor of consistent, more moderate training that you can sustain safely.